diff --git a/src/applications/differential/parser/DifferentialChangesetParser.php b/src/applications/differential/parser/DifferentialChangesetParser.php
index 408988ed1f..3154a8a668 100644
--- a/src/applications/differential/parser/DifferentialChangesetParser.php
+++ b/src/applications/differential/parser/DifferentialChangesetParser.php
@@ -1,1805 +1,1809 @@
 <?php
 
 final class DifferentialChangesetParser extends Phobject {
 
   const HIGHLIGHT_BYTE_LIMIT = 262144;
 
   protected $visible      = array();
   protected $new          = array();
   protected $old          = array();
   protected $intra        = array();
   protected $depthOnlyLines = array();
   protected $newRender    = null;
   protected $oldRender    = null;
 
   protected $filename     = null;
   protected $hunkStartLines = array();
 
   protected $comments     = array();
   protected $specialAttributes = array();
 
   protected $changeset;
 
   protected $renderCacheKey = null;
 
   private $handles = array();
   private $user;
 
   private $leftSideChangesetID;
   private $leftSideAttachesToNewFile;
 
   private $rightSideChangesetID;
   private $rightSideAttachesToNewFile;
 
   private $originalLeft;
   private $originalRight;
 
   private $renderingReference;
   private $isSubparser;
 
   private $isTopLevel;
 
   private $coverage;
   private $markupEngine;
   private $highlightErrors;
   private $disableCache;
   private $renderer;
   private $characterEncoding;
   private $highlightAs;
   private $highlightingDisabled;
   private $showEditAndReplyLinks = true;
   private $canMarkDone;
   private $objectOwnerPHID;
   private $offsetMode;
 
   private $rangeStart;
   private $rangeEnd;
   private $mask;
   private $linesOfContext = 8;
 
   private $highlightEngine;
   private $viewer;
   private $documentEngineKey;
 
   public function setRange($start, $end) {
     $this->rangeStart = $start;
     $this->rangeEnd = $end;
     return $this;
   }
 
   public function setMask(array $mask) {
     $this->mask = $mask;
     return $this;
   }
 
   public function renderChangeset() {
     return $this->render($this->rangeStart, $this->rangeEnd, $this->mask);
   }
 
   public function setShowEditAndReplyLinks($bool) {
     $this->showEditAndReplyLinks = $bool;
     return $this;
   }
 
   public function getShowEditAndReplyLinks() {
     return $this->showEditAndReplyLinks;
   }
 
   public function setHighlightAs($highlight_as) {
     $this->highlightAs = $highlight_as;
     return $this;
   }
 
   public function getHighlightAs() {
     return $this->highlightAs;
   }
 
   public function setCharacterEncoding($character_encoding) {
     $this->characterEncoding = $character_encoding;
     return $this;
   }
 
   public function getCharacterEncoding() {
     return $this->characterEncoding;
   }
 
   public function setRenderer(DifferentialChangesetRenderer $renderer) {
     $this->renderer = $renderer;
     return $this;
   }
 
   public function getRenderer() {
     if (!$this->renderer) {
       return new DifferentialChangesetTwoUpRenderer();
     }
     return $this->renderer;
   }
 
   public function setDisableCache($disable_cache) {
     $this->disableCache = $disable_cache;
     return $this;
   }
 
   public function getDisableCache() {
     return $this->disableCache;
   }
 
   public function setCanMarkDone($can_mark_done) {
     $this->canMarkDone = $can_mark_done;
     return $this;
   }
 
   public function getCanMarkDone() {
     return $this->canMarkDone;
   }
 
   public function setObjectOwnerPHID($phid) {
     $this->objectOwnerPHID = $phid;
     return $this;
   }
 
   public function getObjectOwnerPHID() {
     return $this->objectOwnerPHID;
   }
 
   public function setOffsetMode($offset_mode) {
     $this->offsetMode = $offset_mode;
     return $this;
   }
 
   public function getOffsetMode() {
     return $this->offsetMode;
   }
 
   public function setViewer(PhabricatorUser $viewer) {
     $this->viewer = $viewer;
     return $this;
   }
 
   public function getViewer() {
     return $this->viewer;
   }
 
   public function setDocumentEngineKey($document_engine_key) {
     $this->documentEngineKey = $document_engine_key;
     return $this;
   }
 
   public function getDocumentEngineKey() {
     return $this->documentEngineKey;
   }
 
   public static function getDefaultRendererForViewer(PhabricatorUser $viewer) {
     $is_unified = $viewer->compareUserSetting(
       PhabricatorUnifiedDiffsSetting::SETTINGKEY,
       PhabricatorUnifiedDiffsSetting::VALUE_ALWAYS_UNIFIED);
 
     if ($is_unified) {
       return '1up';
     }
 
     return null;
   }
 
   public function readParametersFromRequest(AphrontRequest $request) {
     $this->setCharacterEncoding($request->getStr('encoding'));
     $this->setHighlightAs($request->getStr('highlight'));
     $this->setDocumentEngineKey($request->getStr('engine'));
 
     $renderer = null;
 
     // If the viewer prefers unified diffs, always set the renderer to unified.
     // Otherwise, we leave it unspecified and the client will choose a
     // renderer based on the screen size.
 
     if ($request->getStr('renderer')) {
       $renderer = $request->getStr('renderer');
     } else {
       $renderer = self::getDefaultRendererForViewer($request->getViewer());
     }
 
     switch ($renderer) {
       case '1up':
         $this->setRenderer(new DifferentialChangesetOneUpRenderer());
         break;
       default:
         $this->setRenderer(new DifferentialChangesetTwoUpRenderer());
         break;
     }
 
     return $this;
   }
 
   const CACHE_VERSION = 14;
   const CACHE_MAX_SIZE = 8e6;
 
   const ATTR_GENERATED  = 'attr:generated';
   const ATTR_DELETED    = 'attr:deleted';
   const ATTR_UNCHANGED  = 'attr:unchanged';
   const ATTR_MOVEAWAY   = 'attr:moveaway';
 
   public function setOldLines(array $lines) {
     $this->old = $lines;
     return $this;
   }
 
   public function setNewLines(array $lines) {
     $this->new = $lines;
     return $this;
   }
 
   public function setSpecialAttributes(array $attributes) {
     $this->specialAttributes = $attributes;
     return $this;
   }
 
   public function setIntraLineDiffs(array $diffs) {
     $this->intra = $diffs;
     return $this;
   }
 
   public function setDepthOnlyLines(array $lines) {
     $this->depthOnlyLines = $lines;
     return $this;
   }
 
   public function getDepthOnlyLines() {
     return $this->depthOnlyLines;
   }
 
   public function setVisibileLinesMask(array $mask) {
     $this->visible = $mask;
     return $this;
   }
 
   public function setLinesOfContext($lines_of_context) {
     $this->linesOfContext = $lines_of_context;
     return $this;
   }
 
   public function getLinesOfContext() {
     return $this->linesOfContext;
   }
 
 
   /**
    * Configure which Changeset comments added to the right side of the visible
    * diff will be attached to. The ID must be the ID of a real Differential
    * Changeset.
    *
    * The complexity here is that we may show an arbitrary side of an arbitrary
    * changeset as either the left or right part of a diff. This method allows
    * the left and right halves of the displayed diff to be correctly mapped to
    * storage changesets.
    *
    * @param id    The Differential Changeset ID that comments added to the right
    *              side of the visible diff should be attached to.
    * @param bool  If true, attach new comments to the right side of the storage
    *              changeset. Note that this may be false, if the left side of
    *              some storage changeset is being shown as the right side of
    *              a display diff.
    * @return this
    */
   public function setRightSideCommentMapping($id, $is_new) {
     $this->rightSideChangesetID = $id;
     $this->rightSideAttachesToNewFile = $is_new;
     return $this;
   }
 
   /**
    * See setRightSideCommentMapping(), but this sets information for the left
    * side of the display diff.
    */
   public function setLeftSideCommentMapping($id, $is_new) {
     $this->leftSideChangesetID = $id;
     $this->leftSideAttachesToNewFile = $is_new;
     return $this;
   }
 
   public function setOriginals(
     DifferentialChangeset $left,
     DifferentialChangeset $right) {
 
     $this->originalLeft = $left;
     $this->originalRight = $right;
     return $this;
   }
 
   public function diffOriginals() {
     $engine = new PhabricatorDifferenceEngine();
     $changeset = $engine->generateChangesetFromFileContent(
       implode('', mpull($this->originalLeft->getHunks(), 'getChanges')),
       implode('', mpull($this->originalRight->getHunks(), 'getChanges')));
 
     $parser = new DifferentialHunkParser();
 
     return $parser->parseHunksForHighlightMasks(
       $changeset->getHunks(),
       $this->originalLeft->getHunks(),
       $this->originalRight->getHunks());
   }
 
   /**
    * Set a key for identifying this changeset in the render cache. If set, the
    * parser will attempt to use the changeset render cache, which can improve
    * performance for frequently-viewed changesets.
    *
    * By default, there is no render cache key and parsers do not use the cache.
    * This is appropriate for rarely-viewed changesets.
    *
    * NOTE: Currently, this key must be a valid Differential Changeset ID.
    *
    * @param   string  Key for identifying this changeset in the render cache.
    * @return  this
    */
   public function setRenderCacheKey($key) {
     $this->renderCacheKey = $key;
     return $this;
   }
 
   private function getRenderCacheKey() {
     return $this->renderCacheKey;
   }
 
   public function setChangeset(DifferentialChangeset $changeset) {
     $this->changeset = $changeset;
 
     $this->setFilename($changeset->getFilename());
 
     return $this;
   }
 
   public function setRenderingReference($ref) {
     $this->renderingReference = $ref;
     return $this;
   }
 
   private function getRenderingReference() {
     return $this->renderingReference;
   }
 
   public function getChangeset() {
     return $this->changeset;
   }
 
   public function setFilename($filename) {
     $this->filename = $filename;
     return $this;
   }
 
   public function setHandles(array $handles) {
     assert_instances_of($handles, 'PhabricatorObjectHandle');
     $this->handles = $handles;
     return $this;
   }
 
   public function setMarkupEngine(PhabricatorMarkupEngine $engine) {
     $this->markupEngine = $engine;
     return $this;
   }
 
   public function setUser(PhabricatorUser $user) {
     $this->user = $user;
     return $this;
   }
 
   public function getUser() {
     return $this->user;
   }
 
   public function setCoverage($coverage) {
     $this->coverage = $coverage;
     return $this;
   }
   private function getCoverage() {
     return $this->coverage;
   }
 
   public function parseInlineComment(
     PhabricatorInlineCommentInterface $comment) {
 
     // Parse only comments which are actually visible.
     if ($this->isCommentVisibleOnRenderedDiff($comment)) {
       $this->comments[] = $comment;
     }
     return $this;
   }
 
   private function loadCache() {
     $render_cache_key = $this->getRenderCacheKey();
     if (!$render_cache_key) {
       return false;
     }
 
     $data = null;
 
     $changeset = new DifferentialChangeset();
     $conn_r = $changeset->establishConnection('r');
     $data = queryfx_one(
       $conn_r,
       'SELECT * FROM %T WHERE id = %d',
       $changeset->getTableName().'_parse_cache',
       $render_cache_key);
 
     if (!$data) {
       return false;
     }
 
     if ($data['cache'][0] == '{') {
       // This is likely an old-style JSON cache which we will not be able to
       // deserialize.
       return false;
     }
 
     $data = unserialize($data['cache']);
     if (!is_array($data) || !$data) {
       return false;
     }
 
     foreach (self::getCacheableProperties() as $cache_key) {
       if (!array_key_exists($cache_key, $data)) {
         // If we're missing a cache key, assume we're looking at an old cache
         // and ignore it.
         return false;
       }
     }
 
     if ($data['cacheVersion'] !== self::CACHE_VERSION) {
       return false;
     }
 
     // Someone displays contents of a partially cached shielded file.
     if (!isset($data['newRender']) && (!$this->isTopLevel || $this->comments)) {
       return false;
     }
 
     unset($data['cacheVersion'], $data['cacheHost']);
     $cache_prop = array_select_keys($data, self::getCacheableProperties());
     foreach ($cache_prop as $cache_key => $v) {
       $this->$cache_key = $v;
     }
 
     return true;
   }
 
   protected static function getCacheableProperties() {
     return array(
       'visible',
       'new',
       'old',
       'intra',
       'depthOnlyLines',
       'newRender',
       'oldRender',
       'specialAttributes',
       'hunkStartLines',
       'cacheVersion',
       'cacheHost',
       'highlightingDisabled',
     );
   }
 
   public function saveCache() {
     if (PhabricatorEnv::isReadOnly()) {
       return false;
     }
 
     if ($this->highlightErrors) {
       return false;
     }
 
     $render_cache_key = $this->getRenderCacheKey();
     if (!$render_cache_key) {
       return false;
     }
 
     $cache = array();
     foreach (self::getCacheableProperties() as $cache_key) {
       switch ($cache_key) {
         case 'cacheVersion':
           $cache[$cache_key] = self::CACHE_VERSION;
           break;
         case 'cacheHost':
           $cache[$cache_key] = php_uname('n');
           break;
         default:
           $cache[$cache_key] = $this->$cache_key;
           break;
       }
     }
     $cache = serialize($cache);
 
     // We don't want to waste too much space by a single changeset.
     if (strlen($cache) > self::CACHE_MAX_SIZE) {
       return;
     }
 
     $changeset = new DifferentialChangeset();
     $conn_w = $changeset->establishConnection('w');
 
     $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
       try {
         queryfx(
           $conn_w,
           'INSERT INTO %T (id, cache, dateCreated) VALUES (%d, %B, %d)
             ON DUPLICATE KEY UPDATE cache = VALUES(cache)',
           DifferentialChangeset::TABLE_CACHE,
           $render_cache_key,
           $cache,
           time());
       } catch (AphrontQueryException $ex) {
         // Ignore these exceptions. A common cause is that the cache is
         // larger than 'max_allowed_packet', in which case we're better off
         // not writing it.
 
         // TODO: It would be nice to tailor this more narrowly.
       }
     unset($unguarded);
   }
 
   private function markGenerated($new_corpus_block = '') {
     $generated_guess = (strpos($new_corpus_block, '@'.'generated') !== false);
 
     if (!$generated_guess) {
       $generated_path_regexps = PhabricatorEnv::getEnvConfig(
         'differential.generated-paths');
       foreach ($generated_path_regexps as $regexp) {
         if (preg_match($regexp, $this->changeset->getFilename())) {
           $generated_guess = true;
           break;
         }
       }
     }
 
     $event = new PhabricatorEvent(
       PhabricatorEventType::TYPE_DIFFERENTIAL_WILLMARKGENERATED,
       array(
         'corpus' => $new_corpus_block,
         'is_generated' => $generated_guess,
       )
     );
     PhutilEventEngine::dispatchEvent($event);
 
     $generated = $event->getValue('is_generated');
 
     $attribute = $this->changeset->isGeneratedChangeset();
     if ($attribute) {
       $generated = true;
     }
 
     $this->specialAttributes[self::ATTR_GENERATED] = $generated;
   }
 
   public function isGenerated() {
     return idx($this->specialAttributes, self::ATTR_GENERATED, false);
   }
 
   public function isDeleted() {
     return idx($this->specialAttributes, self::ATTR_DELETED, false);
   }
 
   public function isUnchanged() {
     return idx($this->specialAttributes, self::ATTR_UNCHANGED, false);
   }
 
   public function isMoveAway() {
     return idx($this->specialAttributes, self::ATTR_MOVEAWAY, false);
   }
 
   private function applyIntraline(&$render, $intra, $corpus) {
 
     foreach ($render as $key => $text) {
       $result = $text;
 
       if (isset($intra[$key])) {
         $result = ArcanistDiffUtils::applyIntralineDiff(
           $result,
           $intra[$key]);
       }
 
       $result = $this->adjustRenderedLineForDisplay($result);
 
       $render[$key] = $result;
     }
   }
 
   private function getHighlightFuture($corpus) {
     $language = $this->highlightAs;
 
     if (!$language) {
       $language = $this->highlightEngine->getLanguageFromFilename(
         $this->filename);
 
       if (($language != 'txt') &&
           (strlen($corpus) > self::HIGHLIGHT_BYTE_LIMIT)) {
         $this->highlightingDisabled = true;
         $language = 'txt';
       }
     }
 
     return $this->highlightEngine->getHighlightFuture(
       $language,
       $corpus);
   }
 
   protected function processHighlightedSource($data, $result) {
 
     $result_lines = phutil_split_lines($result);
     foreach ($data as $key => $info) {
       if (!$info) {
         unset($result_lines[$key]);
       }
     }
     return $result_lines;
   }
 
   private function tryCacheStuff() {
     $skip_cache = false;
 
     if ($this->disableCache) {
       $skip_cache = true;
     }
 
     if ($this->characterEncoding) {
       $skip_cache = true;
     }
 
     if ($this->highlightAs) {
       $skip_cache = true;
     }
 
     $changeset = $this->changeset;
 
     if ($changeset->getFileType() != DifferentialChangeType::FILE_TEXT &&
         $changeset->getFileType() != DifferentialChangeType::FILE_SYMLINK) {
 
       $this->markGenerated();
 
     } else {
       if ($skip_cache || !$this->loadCache()) {
         $this->process();
         if (!$skip_cache) {
           $this->saveCache();
         }
       }
     }
   }
 
   private function process() {
     $changeset = $this->changeset;
 
     $hunk_parser = new DifferentialHunkParser();
     $hunk_parser->parseHunksForLineData($changeset->getHunks());
 
     $this->realignDiff($changeset, $hunk_parser);
 
     $hunk_parser->reparseHunksForSpecialAttributes();
 
     $unchanged = false;
     if (!$hunk_parser->getHasAnyChanges()) {
       $filetype = $this->changeset->getFileType();
       if ($filetype == DifferentialChangeType::FILE_TEXT ||
           $filetype == DifferentialChangeType::FILE_SYMLINK) {
         $unchanged = true;
       }
     }
 
     $moveaway = false;
     $changetype = $this->changeset->getChangeType();
     if ($changetype == DifferentialChangeType::TYPE_MOVE_AWAY) {
       $moveaway = true;
     }
 
     $this->setSpecialAttributes(array(
       self::ATTR_UNCHANGED  => $unchanged,
       self::ATTR_DELETED    => $hunk_parser->getIsDeleted(),
       self::ATTR_MOVEAWAY   => $moveaway,
     ));
 
     $lines_context = $this->getLinesOfContext();
 
     $hunk_parser->generateIntraLineDiffs();
     $hunk_parser->generateVisibileLinesMask($lines_context);
 
     $this->setOldLines($hunk_parser->getOldLines());
     $this->setNewLines($hunk_parser->getNewLines());
     $this->setIntraLineDiffs($hunk_parser->getIntraLineDiffs());
     $this->setDepthOnlyLines($hunk_parser->getDepthOnlyLines());
     $this->setVisibileLinesMask($hunk_parser->getVisibleLinesMask());
     $this->hunkStartLines = $hunk_parser->getHunkStartLines(
       $changeset->getHunks());
 
     $new_corpus = $hunk_parser->getNewCorpus();
     $new_corpus_block = implode('', $new_corpus);
     $this->markGenerated($new_corpus_block);
 
     if ($this->isTopLevel &&
         !$this->comments &&
           ($this->isGenerated() ||
            $this->isUnchanged() ||
            $this->isDeleted())) {
       return;
     }
 
     $old_corpus = $hunk_parser->getOldCorpus();
     $old_corpus_block = implode('', $old_corpus);
     $old_future = $this->getHighlightFuture($old_corpus_block);
     $new_future = $this->getHighlightFuture($new_corpus_block);
     $futures = array(
       'old' => $old_future,
       'new' => $new_future,
     );
     $corpus_blocks = array(
       'old' => $old_corpus_block,
       'new' => $new_corpus_block,
     );
 
     $this->highlightErrors = false;
     foreach (new FutureIterator($futures) as $key => $future) {
       try {
         try {
           $highlighted = $future->resolve();
         } catch (PhutilSyntaxHighlighterException $ex) {
           $this->highlightErrors = true;
           $highlighted = id(new PhutilDefaultSyntaxHighlighter())
             ->getHighlightFuture($corpus_blocks[$key])
             ->resolve();
         }
         switch ($key) {
         case 'old':
           $this->oldRender = $this->processHighlightedSource(
             $this->old,
             $highlighted);
           break;
         case 'new':
           $this->newRender = $this->processHighlightedSource(
             $this->new,
             $highlighted);
           break;
         }
       } catch (Exception $ex) {
         phlog($ex);
         throw $ex;
       }
     }
 
     $this->applyIntraline(
       $this->oldRender,
       ipull($this->intra, 0),
       $old_corpus);
     $this->applyIntraline(
       $this->newRender,
       ipull($this->intra, 1),
       $new_corpus);
   }
 
   private function shouldRenderPropertyChangeHeader($changeset) {
     if (!$this->isTopLevel) {
       // We render properties only at top level; otherwise we get multiple
       // copies of them when a user clicks "Show More".
       return false;
     }
 
     return true;
   }
 
   public function render(
     $range_start  = null,
     $range_len    = null,
     $mask_force   = array()) {
 
     // "Top level" renders are initial requests for the whole file, versus
     // requests for a specific range generated by clicking "show more". We
     // generate property changes and "shield" UI elements only for toplevel
     // requests.
     $this->isTopLevel = (($range_start === null) && ($range_len === null));
     $this->highlightEngine = PhabricatorSyntaxHighlighter::newEngine();
 
     $encoding = null;
     if ($this->characterEncoding) {
       // We are forcing this changeset to be interpreted with a specific
       // character encoding, so force all the hunks into that encoding and
       // propagate it to the renderer.
       $encoding = $this->characterEncoding;
       foreach ($this->changeset->getHunks() as $hunk) {
         $hunk->forceEncoding($this->characterEncoding);
       }
     } else {
       // We're just using the default, so tell the renderer what that is
       // (by reading the encoding from the first hunk).
       foreach ($this->changeset->getHunks() as $hunk) {
         $encoding = $hunk->getDataEncoding();
         break;
       }
     }
 
     $this->tryCacheStuff();
 
     // If we're rendering in an offset mode, treat the range numbers as line
     // numbers instead of rendering offsets.
     $offset_mode = $this->getOffsetMode();
     if ($offset_mode) {
       if ($offset_mode == 'new') {
         $offset_map = $this->new;
       } else {
         $offset_map = $this->old;
       }
 
       // NOTE: Inline comments use zero-based lengths. For example, a comment
       // that starts and ends on line 123 has length 0. Rendering considers
       // this range to have length 1. Probably both should agree, but that
       // ship likely sailed long ago. Tweak things here to get the two systems
       // to agree. See PHI985, where this affected mail rendering of inline
       // comments left on the final line of a file.
 
       $range_end = $this->getOffset($offset_map, $range_start + $range_len);
       $range_start = $this->getOffset($offset_map, $range_start);
       $range_len = ($range_end - $range_start) + 1;
     }
 
     $render_pch = $this->shouldRenderPropertyChangeHeader($this->changeset);
 
     $rows = max(
       count($this->old),
       count($this->new));
 
     $renderer = $this->getRenderer()
       ->setUser($this->getUser())
       ->setChangeset($this->changeset)
       ->setRenderPropertyChangeHeader($render_pch)
       ->setIsTopLevel($this->isTopLevel)
       ->setOldRender($this->oldRender)
       ->setNewRender($this->newRender)
       ->setHunkStartLines($this->hunkStartLines)
       ->setOldChangesetID($this->leftSideChangesetID)
       ->setNewChangesetID($this->rightSideChangesetID)
       ->setOldAttachesToNewFile($this->leftSideAttachesToNewFile)
       ->setNewAttachesToNewFile($this->rightSideAttachesToNewFile)
       ->setCodeCoverage($this->getCoverage())
       ->setRenderingReference($this->getRenderingReference())
       ->setMarkupEngine($this->markupEngine)
       ->setHandles($this->handles)
       ->setOldLines($this->old)
       ->setNewLines($this->new)
       ->setOriginalCharacterEncoding($encoding)
       ->setShowEditAndReplyLinks($this->getShowEditAndReplyLinks())
       ->setCanMarkDone($this->getCanMarkDone())
       ->setObjectOwnerPHID($this->getObjectOwnerPHID())
       ->setHighlightingDisabled($this->highlightingDisabled)
       ->setDepthOnlyLines($this->getDepthOnlyLines());
 
+    $engine_blocks = $this->newDocumentEngineBlocks();
+    $has_document_engine = ($engine_blocks !== null);
+
     $shield = null;
-    if ($this->isTopLevel && !$this->comments) {
+    if ($this->isTopLevel && !$this->comments && !$has_document_engine) {
       if ($this->isGenerated()) {
         $shield = $renderer->renderShield(
           pht(
             'This file contains generated code, which does not normally '.
             'need to be reviewed.'));
       } else if ($this->isMoveAway()) {
         // We put an empty shield on these files. Normally, they do not have
         // any diff content anyway. However, if they come through `arc`, they
         // may have content. We don't want to show it (it's not useful) and
         // we bailed out of fully processing it earlier anyway.
 
         // We could show a message like "this file was moved", but we show
         // that as a change header anyway, so it would be redundant. Instead,
         // just render an empty shield to skip rendering the diff body.
         $shield = '';
       } else if ($this->isUnchanged()) {
         $type = 'text';
         if (!$rows) {
           // NOTE: Normally, diffs which don't change files do not include
           // file content (for example, if you "chmod +x" a file and then
           // run "git show", the file content is not available). Similarly,
           // if you move a file from A to B without changing it, diffs normally
           // do not show the file content. In some cases `arc` is able to
           // synthetically generate content for these diffs, but for raw diffs
           // we'll never have it so we need to be prepared to not render a link.
           $type = 'none';
         }
 
         $type_add = DifferentialChangeType::TYPE_ADD;
         if ($this->changeset->getChangeType() == $type_add) {
           // Although the generic message is sort of accurate in a technical
           // sense, this more-tailored message is less confusing.
           $shield = $renderer->renderShield(
             pht('This is an empty file.'),
             $type);
         } else {
           $shield = $renderer->renderShield(
             pht('The contents of this file were not changed.'),
             $type);
         }
       } else if ($this->isDeleted()) {
         $shield = $renderer->renderShield(
           pht('This file was completely deleted.'));
       } else if ($this->changeset->getAffectedLineCount() > 2500) {
         $shield = $renderer->renderShield(
           pht(
             'This file has a very large number of changes (%s lines).',
             new PhutilNumber($this->changeset->getAffectedLineCount())));
       }
     }
 
     if ($shield !== null) {
       return $renderer->renderChangesetTable($shield);
     }
 
     // This request should render the "undershield" headers if it's a top-level
     // request which made it this far (indicating the changeset has no shield)
     // or it's a request with no mask information (indicating it's the request
     // that removes the rendering shield). Possibly, this second class of
     // request might need to be made more explicit.
     $is_undershield = (empty($mask_force) || $this->isTopLevel);
     $renderer->setIsUndershield($is_undershield);
 
     $old_comments = array();
     $new_comments = array();
     $old_mask = array();
     $new_mask = array();
     $feedback_mask = array();
     $lines_context = $this->getLinesOfContext();
 
     if ($this->comments) {
       // If there are any comments which appear in sections of the file which
       // we don't have, we're going to move them backwards to the closest
       // earlier line. Two cases where this may happen are:
       //
       //   - Porting ghost comments forward into a file which was mostly
       //     deleted.
       //   - Porting ghost comments forward from a full-context diff to a
       //     partial-context diff.
 
       list($old_backmap, $new_backmap) = $this->buildLineBackmaps();
 
       foreach ($this->comments as $comment) {
         $new_side = $this->isCommentOnRightSideWhenDisplayed($comment);
 
         $line = $comment->getLineNumber();
         if ($new_side) {
           $back_line = $new_backmap[$line];
         } else {
           $back_line = $old_backmap[$line];
         }
 
         if ($back_line != $line) {
           // TODO: This should probably be cleaner, but just be simple and
           // obvious for now.
           $ghost = $comment->getIsGhost();
           if ($ghost) {
             $moved = pht(
               'This comment originally appeared on line %s, but that line '.
               'does not exist in this version of the diff. It has been '.
               'moved backward to the nearest line.',
               new PhutilNumber($line));
             $ghost['reason'] = $ghost['reason']."\n\n".$moved;
             $comment->setIsGhost($ghost);
           }
 
           $comment->setLineNumber($back_line);
           $comment->setLineLength(0);
 
         }
 
         $start = max($comment->getLineNumber() - $lines_context, 0);
         $end = $comment->getLineNumber() +
           $comment->getLineLength() +
           $lines_context;
         for ($ii = $start; $ii <= $end; $ii++) {
           if ($new_side) {
             $new_mask[$ii] = true;
           } else {
             $old_mask[$ii] = true;
           }
         }
       }
 
       foreach ($this->old as $ii => $old) {
         if (isset($old['line']) && isset($old_mask[$old['line']])) {
           $feedback_mask[$ii] = true;
         }
       }
 
       foreach ($this->new as $ii => $new) {
         if (isset($new['line']) && isset($new_mask[$new['line']])) {
           $feedback_mask[$ii] = true;
         }
       }
 
       $this->comments = id(new PHUIDiffInlineThreader())
         ->reorderAndThreadCommments($this->comments);
 
       foreach ($this->comments as $comment) {
         $final = $comment->getLineNumber() +
           $comment->getLineLength();
         $final = max(1, $final);
         if ($this->isCommentOnRightSideWhenDisplayed($comment)) {
           $new_comments[$final][] = $comment;
         } else {
           $old_comments[$final][] = $comment;
         }
       }
     }
     $renderer
       ->setOldComments($old_comments)
       ->setNewComments($new_comments);
 
-    $engine_blocks = $this->newDocumentEngineChangesetView();
     if ($engine_blocks !== null) {
       $reference = $this->getRenderingReference();
       $parts = explode('/', $reference);
       if (count($parts) == 2) {
         list($id, $vs) = $parts;
       } else {
         $id = $parts[0];
         $vs = 0;
       }
 
       // If we don't have an explicit "vs" changeset, it's the left side of
       // the "id" changeset.
       if (!$vs) {
         $vs = $id;
       }
 
+      $renderer->setDocumentEngineBlocks($engine_blocks);
+
       return $renderer->renderDocumentEngineBlocks(
         $engine_blocks,
         (string)$id,
         (string)$vs);
     }
 
     // If we've made it here with a type of file we don't know how to render,
     // bail out with a default empty rendering. Normally, we'd expect a
     // document engine to catch these changes before we make it this far.
     switch ($this->changeset->getFileType()) {
       case DifferentialChangeType::FILE_DIRECTORY:
       case DifferentialChangeType::FILE_BINARY:
       case DifferentialChangeType::FILE_IMAGE:
         $output = $renderer->renderChangesetTable(null);
         return $output;
     }
 
     if ($this->originalLeft && $this->originalRight) {
       list($highlight_old, $highlight_new) = $this->diffOriginals();
       $highlight_old = array_flip($highlight_old);
       $highlight_new = array_flip($highlight_new);
       $renderer
         ->setHighlightOld($highlight_old)
         ->setHighlightNew($highlight_new);
     }
     $renderer
       ->setOriginalOld($this->originalLeft)
       ->setOriginalNew($this->originalRight);
 
     if ($range_start === null) {
       $range_start = 0;
     }
     if ($range_len === null) {
       $range_len = $rows;
     }
     $range_len = min($range_len, $rows - $range_start);
 
     list($gaps, $mask) = $this->calculateGapsAndMask(
       $mask_force,
       $feedback_mask,
       $range_start,
       $range_len);
 
     $renderer
       ->setGaps($gaps)
       ->setMask($mask);
 
     $html = $renderer->renderTextChange(
       $range_start,
       $range_len,
       $rows);
 
     return $renderer->renderChangesetTable($html);
   }
 
   /**
    * This function calculates a lot of stuff we need to know to display
    * the diff:
    *
    * Gaps - compute gaps in the visible display diff, where we will render
    * "Show more context" spacers. If a gap is smaller than the context size,
    * we just display it. Otherwise, we record it into $gaps and will render a
    * "show more context" element instead of diff text below. A given $gap
    * is a tuple of $gap_line_number_start and $gap_length.
    *
    * Mask - compute the actual lines that need to be shown (because they
    * are near changes lines, near inline comments, or the request has
    * explicitly asked for them, i.e. resulting from the user clicking
    * "show more"). The $mask returned is a sparsely populated dictionary
    * of $visible_line_number => true.
    *
    * @return array($gaps, $mask)
    */
   private function calculateGapsAndMask(
     $mask_force,
     $feedback_mask,
     $range_start,
     $range_len) {
 
     $lines_context = $this->getLinesOfContext();
 
     $gaps = array();
     $gap_start = 0;
     $in_gap = false;
     $base_mask = $this->visible + $mask_force + $feedback_mask;
     $base_mask[$range_start + $range_len] = true;
     for ($ii = $range_start; $ii <= $range_start + $range_len; $ii++) {
       if (isset($base_mask[$ii])) {
         if ($in_gap) {
           $gap_length = $ii - $gap_start;
           if ($gap_length <= $lines_context) {
             for ($jj = $gap_start; $jj <= $gap_start + $gap_length; $jj++) {
               $base_mask[$jj] = true;
             }
           } else {
             $gaps[] = array($gap_start, $gap_length);
           }
           $in_gap = false;
         }
       } else {
         if (!$in_gap) {
           $gap_start = $ii;
           $in_gap = true;
         }
       }
     }
     $gaps = array_reverse($gaps);
     $mask = $base_mask;
 
     return array($gaps, $mask);
   }
 
   /**
    * Determine if an inline comment will appear on the rendered diff,
    * taking into consideration which halves of which changesets will actually
    * be shown.
    *
    * @param PhabricatorInlineCommentInterface Comment to test for visibility.
    * @return bool True if the comment is visible on the rendered diff.
    */
   private function isCommentVisibleOnRenderedDiff(
     PhabricatorInlineCommentInterface $comment) {
 
     $changeset_id = $comment->getChangesetID();
     $is_new = $comment->getIsNewFile();
 
     if ($changeset_id == $this->rightSideChangesetID &&
         $is_new == $this->rightSideAttachesToNewFile) {
         return true;
     }
 
     if ($changeset_id == $this->leftSideChangesetID &&
         $is_new == $this->leftSideAttachesToNewFile) {
         return true;
     }
 
     return false;
   }
 
 
   /**
    * Determine if a comment will appear on the right side of the display diff.
    * Note that the comment must appear somewhere on the rendered changeset, as
    * per isCommentVisibleOnRenderedDiff().
    *
    * @param PhabricatorInlineCommentInterface Comment to test for display
    *              location.
    * @return bool True for right, false for left.
    */
   private function isCommentOnRightSideWhenDisplayed(
     PhabricatorInlineCommentInterface $comment) {
 
     if (!$this->isCommentVisibleOnRenderedDiff($comment)) {
       throw new Exception(pht('Comment is not visible on changeset!'));
     }
 
     $changeset_id = $comment->getChangesetID();
     $is_new = $comment->getIsNewFile();
 
     if ($changeset_id == $this->rightSideChangesetID &&
         $is_new == $this->rightSideAttachesToNewFile) {
       return true;
     }
 
     return false;
   }
 
   /**
    * Parse the 'range' specification that this class and the client-side JS
    * emit to indicate that a user clicked "Show more..." on a diff. Generally,
    * use is something like this:
    *
    *   $spec = $request->getStr('range');
    *   $parsed = DifferentialChangesetParser::parseRangeSpecification($spec);
    *   list($start, $end, $mask) = $parsed;
    *   $parser->render($start, $end, $mask);
    *
    * @param string Range specification, indicating the range of the diff that
    *               should be rendered.
    * @return tuple List of <start, end, mask> suitable for passing to
    *               @{method:render}.
    */
   public static function parseRangeSpecification($spec) {
     $range_s = null;
     $range_e = null;
     $mask = array();
 
     if ($spec) {
       $match = null;
       if (preg_match('@^(\d+)-(\d+)(?:/(\d+)-(\d+))?$@', $spec, $match)) {
         $range_s = (int)$match[1];
         $range_e = (int)$match[2];
         if (count($match) > 3) {
           $start = (int)$match[3];
           $len = (int)$match[4];
           for ($ii = $start; $ii < $start + $len; $ii++) {
             $mask[$ii] = true;
           }
         }
       }
     }
 
     return array($range_s, $range_e, $mask);
   }
 
   /**
    * Render "modified coverage" information; test coverage on modified lines.
    * This synthesizes diff information with unit test information into a useful
    * indicator of how well tested a change is.
    */
   public function renderModifiedCoverage() {
     $na = phutil_tag('em', array(), '-');
 
     $coverage = $this->getCoverage();
     if (!$coverage) {
       return $na;
     }
 
     $covered = 0;
     $not_covered = 0;
 
     foreach ($this->new as $k => $new) {
       if (!$new['line']) {
         continue;
       }
 
       if (!$new['type']) {
         continue;
       }
 
       if (empty($coverage[$new['line'] - 1])) {
         continue;
       }
 
       switch ($coverage[$new['line'] - 1]) {
         case 'C':
           $covered++;
           break;
         case 'U':
           $not_covered++;
           break;
       }
     }
 
     if (!$covered && !$not_covered) {
       return $na;
     }
 
     return sprintf('%d%%', 100 * ($covered / ($covered + $not_covered)));
   }
 
   /**
    * Build maps from lines comments appear on to actual lines.
    */
   private function buildLineBackmaps() {
     $old_back = array();
     $new_back = array();
     foreach ($this->old as $ii => $old) {
       $old_back[$old['line']] = $old['line'];
     }
     foreach ($this->new as $ii => $new) {
       $new_back[$new['line']] = $new['line'];
     }
 
     $max_old_line = 0;
     $max_new_line = 0;
     foreach ($this->comments as $comment) {
       if ($this->isCommentOnRightSideWhenDisplayed($comment)) {
         $max_new_line = max($max_new_line, $comment->getLineNumber());
       } else {
         $max_old_line = max($max_old_line, $comment->getLineNumber());
       }
     }
 
     $cursor = 1;
     for ($ii = 1; $ii <= $max_old_line; $ii++) {
       if (empty($old_back[$ii])) {
         $old_back[$ii] = $cursor;
       } else {
         $cursor = $old_back[$ii];
       }
     }
 
     $cursor = 1;
     for ($ii = 1; $ii <= $max_new_line; $ii++) {
       if (empty($new_back[$ii])) {
         $new_back[$ii] = $cursor;
       } else {
         $cursor = $new_back[$ii];
       }
     }
 
     return array($old_back, $new_back);
   }
 
   private function getOffset(array $map, $line) {
     if (!$map) {
       return null;
     }
 
     $line = (int)$line;
     foreach ($map as $key => $spec) {
       if ($spec && isset($spec['line'])) {
         if ((int)$spec['line'] >= $line) {
           return $key;
         }
       }
     }
 
     return $key;
   }
 
   private function realignDiff(
     DifferentialChangeset $changeset,
     DifferentialHunkParser $hunk_parser) {
     // Normalizing and realigning the diff depends on rediffing the files, and
     // we currently need complete representations of both files to do anything
     // reasonable. If we only have parts of the files, skip realignment.
 
     // We have more than one hunk, so we're definitely missing part of the file.
     $hunks = $changeset->getHunks();
     if (count($hunks) !== 1) {
       return null;
     }
 
     // The first hunk doesn't start at the beginning of the file, so we're
     // missing some context.
     $first_hunk = head($hunks);
     if ($first_hunk->getOldOffset() != 1 || $first_hunk->getNewOffset() != 1) {
       return null;
     }
 
     $old_file = $changeset->makeOldFile();
     $new_file = $changeset->makeNewFile();
     if ($old_file === $new_file) {
       // If the old and new files are exactly identical, the synthetic
       // diff below will give us nonsense and whitespace modes are
       // irrelevant anyway. This occurs when you, e.g., copy a file onto
       // itself in Subversion (see T271).
       return null;
     }
 
 
     $engine = id(new PhabricatorDifferenceEngine())
       ->setNormalize(true);
 
     $normalized_changeset = $engine->generateChangesetFromFileContent(
       $old_file,
       $new_file);
 
     $type_parser = new DifferentialHunkParser();
     $type_parser->parseHunksForLineData($normalized_changeset->getHunks());
 
     $hunk_parser->setNormalized(true);
     $hunk_parser->setOldLineTypeMap($type_parser->getOldLineTypeMap());
     $hunk_parser->setNewLineTypeMap($type_parser->getNewLineTypeMap());
   }
 
   private function adjustRenderedLineForDisplay($line) {
     // IMPORTANT: We're using "str_replace()" against raw HTML here, which can
     // easily become unsafe. The input HTML has already had syntax highlighting
     // and intraline diff highlighting applied, so it's full of "<span />" tags.
 
     static $search;
     static $replace;
     if ($search === null) {
       $rules = $this->newSuspiciousCharacterRules();
 
       $map = array();
       foreach ($rules as $key => $spec) {
         $tag = phutil_tag(
           'span',
           array(
             'data-copy-text' => $key,
             'class' => $spec['class'],
             'title' => $spec['title'],
           ),
           $spec['replacement']);
         $map[$key] = phutil_string_cast($tag);
       }
 
       $search = array_keys($map);
       $replace = array_values($map);
     }
 
     $is_html = false;
     if ($line instanceof PhutilSafeHTML) {
       $is_html = true;
       $line = hsprintf('%s', $line);
     }
 
     $line = phutil_string_cast($line);
 
     // TODO: This should be flexible, eventually.
     $tab_width = 2;
 
     $line = self::replaceTabsWithSpaces($line, $tab_width);
     $line = str_replace($search, $replace, $line);
 
     if ($is_html) {
       $line = phutil_safe_html($line);
     }
 
     return $line;
   }
 
   private function newSuspiciousCharacterRules() {
     // The "title" attributes are cached in the database, so they're
     // intentionally not wrapped in "pht(...)".
 
     $rules = array(
       "\xE2\x80\x8B" => array(
         'title' => 'ZWS',
         'class' => 'suspicious-character',
         'replacement' => '!',
       ),
       "\xC2\xA0" => array(
         'title' => 'NBSP',
         'class' => 'suspicious-character',
         'replacement' => '!',
       ),
       "\x7F" => array(
         'title' => 'DEL (0x7F)',
         'class' => 'suspicious-character',
         'replacement' => "\xE2\x90\xA1",
       ),
     );
 
     // Unicode defines special pictures for the control characters in the
     // range between "0x00" and "0x1F".
 
     $control = array(
       'NULL',
       'SOH',
       'STX',
       'ETX',
       'EOT',
       'ENQ',
       'ACK',
       'BEL',
       'BS',
       null, // "\t" Tab
       null, // "\n" New Line
       'VT',
       'FF',
       null, // "\r" Carriage Return,
       'SO',
       'SI',
       'DLE',
       'DC1',
       'DC2',
       'DC3',
       'DC4',
       'NAK',
       'SYN',
       'ETB',
       'CAN',
       'EM',
       'SUB',
       'ESC',
       'FS',
       'GS',
       'RS',
       'US',
     );
 
     foreach ($control as $idx => $label) {
       if ($label === null) {
         continue;
       }
 
       $rules[chr($idx)] = array(
         'title' => sprintf('%s (0x%02X)', $label, $idx),
         'class' => 'suspicious-character',
         'replacement' => "\xE2\x90".chr(0x80 + $idx),
       );
     }
 
     return $rules;
   }
 
   public static function replaceTabsWithSpaces($line, $tab_width) {
     static $tags = array();
     if (empty($tags[$tab_width])) {
       for ($ii = 1; $ii <= $tab_width; $ii++) {
         $tag = phutil_tag(
           'span',
           array(
             'data-copy-text' => "\t",
           ),
           str_repeat(' ', $ii));
         $tag = phutil_string_cast($tag);
         $tags[$ii] = $tag;
       }
     }
 
     // Expand all prefix tabs until we encounter any non-tab character. This
     // is cheap and often immediately produces the correct result with no
     // further work (and, particularly, no need to handle any unicode cases).
 
     $len = strlen($line);
 
     $head = 0;
     for ($head = 0; $head < $len; $head++) {
       $char = $line[$head];
       if ($char !== "\t") {
         break;
       }
     }
 
     if ($head) {
       if (empty($tags[$tab_width * $head])) {
         $tags[$tab_width * $head] = str_repeat($tags[$tab_width], $head);
       }
       $prefix = $tags[$tab_width * $head];
       $line = substr($line, $head);
     } else {
       $prefix = '';
     }
 
     // If we have no remaining tabs elsewhere in the string after taking care
     // of all the prefix tabs, we're done.
     if (strpos($line, "\t") === false) {
       return $prefix.$line;
     }
 
     $len = strlen($line);
 
     // If the line is particularly long, don't try to do anything special with
     // it. Use a faster approximation of the correct tabstop expansion instead.
     // This usually still arrives at the right result.
     if ($len > 256) {
       return $prefix.str_replace("\t", $tags[$tab_width], $line);
     }
 
     $in_tag = false;
     $pos = 0;
 
     // See PHI1210. If the line only has single-byte characters, we don't need
     // to vectorize it and can avoid an expensive UTF8 call.
 
     $fast_path = preg_match('/^[\x01-\x7F]*\z/', $line);
     if ($fast_path) {
       $replace = array();
       for ($ii = 0; $ii < $len; $ii++) {
         $char = $line[$ii];
         if ($char === '>') {
           $in_tag = false;
           continue;
         }
 
         if ($in_tag) {
           continue;
         }
 
         if ($char === '<') {
           $in_tag = true;
           continue;
         }
 
         if ($char === "\t") {
           $count = $tab_width - ($pos % $tab_width);
           $pos += $count;
           $replace[$ii] = $tags[$count];
           continue;
         }
 
         $pos++;
       }
 
       if ($replace) {
         // Apply replacements starting at the end of the string so they
         // don't mess up the offsets for following replacements.
         $replace = array_reverse($replace, true);
 
         foreach ($replace as $replace_pos => $replacement) {
           $line = substr_replace($line, $replacement, $replace_pos, 1);
         }
       }
     } else {
       $line = phutil_utf8v_combined($line);
       foreach ($line as $key => $char) {
         if ($char === '>') {
           $in_tag = false;
           continue;
         }
 
         if ($in_tag) {
           continue;
         }
 
         if ($char === '<') {
           $in_tag = true;
           continue;
         }
 
         if ($char === "\t") {
           $count = $tab_width - ($pos % $tab_width);
           $pos += $count;
           $line[$key] = $tags[$count];
           continue;
         }
 
         $pos++;
       }
 
       $line = implode('', $line);
     }
 
     return $prefix.$line;
   }
 
-  private function newDocumentEngineChangesetView() {
+  private function newDocumentEngineBlocks() {
     $changeset = $this->changeset;
     $viewer = $this->getViewer();
 
     // TODO: This should probably be made non-optional in the future.
     if (!$viewer) {
       return null;
     }
 
     $old_file = null;
     $new_file = null;
 
     switch ($changeset->getFileType()) {
       case DifferentialChangeType::FILE_IMAGE:
       case DifferentialChangeType::FILE_BINARY:
         list($old_file, $new_file) = $this->loadFileObjectsForChangeset();
         break;
     }
 
     $old_ref = id(new PhabricatorDocumentRef())
       ->setName($changeset->getOldFile());
     if ($old_file) {
       $old_ref->setFile($old_file);
     } else {
       $old_data = $this->old;
       $old_data = ipull($old_data, 'text');
       $old_data = implode('', $old_data);
 
       $old_ref->setData($old_data);
     }
 
     $new_ref = id(new PhabricatorDocumentRef())
       ->setName($changeset->getFilename());
     if ($new_file) {
       $new_ref->setFile($new_file);
     } else {
       $new_data = $this->new;
       $new_data = ipull($new_data, 'text');
       $new_data = implode('', $new_data);
 
       $new_ref->setData($new_data);
     }
 
     $old_engines = PhabricatorDocumentEngine::getEnginesForRef(
       $viewer,
       $old_ref);
 
     $new_engines = PhabricatorDocumentEngine::getEnginesForRef(
       $viewer,
       $new_ref);
 
     $shared_engines = array_intersect_key($old_engines, $new_engines);
 
     foreach ($shared_engines as $key => $shared_engine) {
       if (!$shared_engine->canDiffDocuments($old_ref, $new_ref)) {
         unset($shared_engines[$key]);
       }
     }
 
     $engine_key = $this->getDocumentEngineKey();
     if (strlen($engine_key)) {
       if (isset($shared_engines[$engine_key])) {
         $document_engine = $shared_engines[$engine_key];
       } else {
         $document_engine = null;
       }
     } else {
       $document_engine = head($shared_engines);
     }
 
     if ($document_engine) {
-      return $document_engine->newDiffView(
+      return $document_engine->newEngineBlocks(
         $old_ref,
         $new_ref);
     }
 
     return null;
   }
 
   private function loadFileObjectsForChangeset() {
     $changeset = $this->changeset;
     $viewer = $this->getViewer();
 
     $old_file = null;
     $new_file = null;
 
     // TODO: Improve the architectural issue as discussed in D955
     // https://secure.phabricator.com/D955
     $reference = $this->getRenderingReference();
     $parts = explode('/', $reference);
     if (count($parts) == 2) {
       list($id, $vs) = $parts;
     } else {
       $id = $parts[0];
       $vs = 0;
     }
     $id = (int)$id;
     $vs = (int)$vs;
 
     if (!$vs) {
       $metadata = $this->changeset->getMetadata();
       $data = idx($metadata, 'attachment-data');
 
       $old_phid = idx($metadata, 'old:binary-phid');
       $new_phid = idx($metadata, 'new:binary-phid');
     } else {
       $vs_changeset = id(new DifferentialChangeset())->load($vs);
       $old_phid = null;
       $new_phid = null;
 
       // TODO: This is spooky, see D6851
       if ($vs_changeset) {
         $vs_metadata = $vs_changeset->getMetadata();
         $old_phid = idx($vs_metadata, 'new:binary-phid');
       }
 
       $changeset = id(new DifferentialChangeset())->load($id);
       if ($changeset) {
         $metadata = $changeset->getMetadata();
         $new_phid = idx($metadata, 'new:binary-phid');
       }
     }
 
     if ($old_phid || $new_phid) {
       $file_phids = array();
       if ($old_phid) {
         $file_phids[] = $old_phid;
       }
       if ($new_phid) {
         $file_phids[] = $new_phid;
       }
 
       $files = id(new PhabricatorFileQuery())
         ->setViewer($viewer)
         ->withPHIDs($file_phids)
         ->execute();
 
       foreach ($files as $file) {
         if ($file->getPHID() == $old_phid) {
           $old_file = $file;
         } else if ($file->getPHID() == $new_phid) {
           $new_file = $file;
         }
       }
     }
 
     return array($old_file, $new_file);
   }
 
 }
diff --git a/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php b/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
index ae8db861b6..c856b9b14d 100644
--- a/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
@@ -1,611 +1,620 @@
 <?php
 
 abstract class DifferentialChangesetHTMLRenderer
   extends DifferentialChangesetRenderer {
 
   public static function getHTMLRendererByKey($key) {
     switch ($key) {
       case '1up':
         return new DifferentialChangesetOneUpRenderer();
       case '2up':
       default:
         return new DifferentialChangesetTwoUpRenderer();
     }
     throw new Exception(pht('Unknown HTML renderer "%s"!', $key));
   }
 
   abstract protected function getRendererTableClass();
   abstract public function getRowScaffoldForInline(
     PHUIDiffInlineCommentView $view);
 
   protected function renderChangeTypeHeader($force) {
     $changeset = $this->getChangeset();
 
     $change = $changeset->getChangeType();
     $file = $changeset->getFileType();
 
     $messages = array();
     switch ($change) {
 
       case DifferentialChangeType::TYPE_ADD:
         switch ($file) {
           case DifferentialChangeType::FILE_TEXT:
             $messages[] = pht('This file was added.');
             break;
           case DifferentialChangeType::FILE_IMAGE:
             $messages[] = pht('This image was added.');
             break;
           case DifferentialChangeType::FILE_DIRECTORY:
             $messages[] = pht('This directory was added.');
             break;
           case DifferentialChangeType::FILE_BINARY:
             $messages[] = pht('This binary file was added.');
             break;
           case DifferentialChangeType::FILE_SYMLINK:
             $messages[] = pht('This symlink was added.');
             break;
           case DifferentialChangeType::FILE_SUBMODULE:
             $messages[] = pht('This submodule was added.');
             break;
         }
         break;
 
       case DifferentialChangeType::TYPE_DELETE:
         switch ($file) {
           case DifferentialChangeType::FILE_TEXT:
             $messages[] = pht('This file was deleted.');
             break;
           case DifferentialChangeType::FILE_IMAGE:
             $messages[] = pht('This image was deleted.');
             break;
           case DifferentialChangeType::FILE_DIRECTORY:
             $messages[] = pht('This directory was deleted.');
             break;
           case DifferentialChangeType::FILE_BINARY:
             $messages[] = pht('This binary file was deleted.');
             break;
           case DifferentialChangeType::FILE_SYMLINK:
             $messages[] = pht('This symlink was deleted.');
             break;
           case DifferentialChangeType::FILE_SUBMODULE:
             $messages[] = pht('This submodule was deleted.');
             break;
         }
         break;
 
       case DifferentialChangeType::TYPE_MOVE_HERE:
         $from = phutil_tag('strong', array(), $changeset->getOldFile());
         switch ($file) {
           case DifferentialChangeType::FILE_TEXT:
             $messages[] = pht('This file was moved from %s.', $from);
             break;
           case DifferentialChangeType::FILE_IMAGE:
             $messages[] = pht('This image was moved from %s.', $from);
             break;
           case DifferentialChangeType::FILE_DIRECTORY:
             $messages[] = pht('This directory was moved from %s.', $from);
             break;
           case DifferentialChangeType::FILE_BINARY:
             $messages[] = pht('This binary file was moved from %s.', $from);
             break;
           case DifferentialChangeType::FILE_SYMLINK:
             $messages[] = pht('This symlink was moved from %s.', $from);
             break;
           case DifferentialChangeType::FILE_SUBMODULE:
             $messages[] = pht('This submodule was moved from %s.', $from);
             break;
         }
         break;
 
       case DifferentialChangeType::TYPE_COPY_HERE:
         $from = phutil_tag('strong', array(), $changeset->getOldFile());
         switch ($file) {
           case DifferentialChangeType::FILE_TEXT:
             $messages[] = pht('This file was copied from %s.', $from);
             break;
           case DifferentialChangeType::FILE_IMAGE:
             $messages[] = pht('This image was copied from %s.', $from);
             break;
           case DifferentialChangeType::FILE_DIRECTORY:
             $messages[] = pht('This directory was copied from %s.', $from);
             break;
           case DifferentialChangeType::FILE_BINARY:
             $messages[] = pht('This binary file was copied from %s.', $from);
             break;
           case DifferentialChangeType::FILE_SYMLINK:
             $messages[] = pht('This symlink was copied from %s.', $from);
             break;
           case DifferentialChangeType::FILE_SUBMODULE:
             $messages[] = pht('This submodule was copied from %s.', $from);
             break;
         }
         break;
 
       case DifferentialChangeType::TYPE_MOVE_AWAY:
         $paths = phutil_tag(
           'strong',
           array(),
           implode(', ', $changeset->getAwayPaths()));
         switch ($file) {
           case DifferentialChangeType::FILE_TEXT:
             $messages[] = pht('This file was moved to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_IMAGE:
             $messages[] = pht('This image was moved to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_DIRECTORY:
             $messages[] = pht('This directory was moved to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_BINARY:
             $messages[] = pht('This binary file was moved to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_SYMLINK:
             $messages[] = pht('This symlink was moved to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_SUBMODULE:
             $messages[] = pht('This submodule was moved to %s.', $paths);
             break;
         }
         break;
 
       case DifferentialChangeType::TYPE_COPY_AWAY:
         $paths = phutil_tag(
           'strong',
           array(),
           implode(', ', $changeset->getAwayPaths()));
         switch ($file) {
           case DifferentialChangeType::FILE_TEXT:
             $messages[] = pht('This file was copied to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_IMAGE:
             $messages[] = pht('This image was copied to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_DIRECTORY:
             $messages[] = pht('This directory was copied to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_BINARY:
             $messages[] = pht('This binary file was copied to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_SYMLINK:
             $messages[] = pht('This symlink was copied to %s.', $paths);
             break;
           case DifferentialChangeType::FILE_SUBMODULE:
             $messages[] = pht('This submodule was copied to %s.', $paths);
             break;
         }
         break;
 
       case DifferentialChangeType::TYPE_MULTICOPY:
         $paths = phutil_tag(
           'strong',
           array(),
           implode(', ', $changeset->getAwayPaths()));
         switch ($file) {
           case DifferentialChangeType::FILE_TEXT:
             $messages[] = pht(
               'This file was deleted after being copied to %s.',
               $paths);
             break;
           case DifferentialChangeType::FILE_IMAGE:
             $messages[] = pht(
               'This image was deleted after being copied to %s.',
               $paths);
             break;
           case DifferentialChangeType::FILE_DIRECTORY:
             $messages[] = pht(
               'This directory was deleted after being copied to %s.',
               $paths);
             break;
           case DifferentialChangeType::FILE_BINARY:
             $messages[] = pht(
               'This binary file was deleted after being copied to %s.',
               $paths);
             break;
           case DifferentialChangeType::FILE_SYMLINK:
             $messages[] = pht(
               'This symlink was deleted after being copied to %s.',
               $paths);
             break;
           case DifferentialChangeType::FILE_SUBMODULE:
             $messages[] = pht(
               'This submodule was deleted after being copied to %s.',
               $paths);
             break;
         }
         break;
 
       default:
         switch ($file) {
           case DifferentialChangeType::FILE_TEXT:
             // This is the default case, so we only render this header if
             // forced to since it's not very useful.
             if ($force) {
               $messages[] = pht('This file was not modified.');
             }
             break;
           case DifferentialChangeType::FILE_IMAGE:
             $messages[] = pht('This is an image.');
             break;
           case DifferentialChangeType::FILE_DIRECTORY:
             $messages[] = pht('This is a directory.');
             break;
           case DifferentialChangeType::FILE_BINARY:
             $messages[] = pht('This is a binary file.');
             break;
           case DifferentialChangeType::FILE_SYMLINK:
             $messages[] = pht('This is a symlink.');
             break;
           case DifferentialChangeType::FILE_SUBMODULE:
             $messages[] = pht('This is a submodule.');
             break;
         }
         break;
     }
 
     return $this->formatHeaderMessages($messages);
   }
 
   protected function renderUndershieldHeader() {
     $messages = array();
 
     $changeset = $this->getChangeset();
 
     $file = $changeset->getFileType();
 
     // If this is a text file with at least one hunk, we may have converted
     // the text encoding. In this case, show a note.
     $show_encoding = ($file == DifferentialChangeType::FILE_TEXT) &&
                      ($changeset->getHunks());
 
     if ($show_encoding) {
       $encoding = $this->getOriginalCharacterEncoding();
       if ($encoding != 'utf8') {
         if ($encoding) {
           $messages[] = pht(
             'This file was converted from %s for display.',
             phutil_tag('strong', array(), $encoding));
         } else {
           $messages[] = pht('This file uses an unknown character encoding.');
         }
       }
     }
 
-    if ($this->getHighlightingDisabled()) {
-      $messages[] = pht(
-        'This file is larger than %s, so syntax highlighting is '.
-        'disabled by default.',
-        phutil_format_bytes(DifferentialChangesetParser::HIGHLIGHT_BYTE_LIMIT));
+    $blocks = $this->getDocumentEngineBlocks();
+    if ($blocks) {
+      foreach ($blocks->getMessages() as $message) {
+        $messages[] = $message;
+      }
+    } else {
+      if ($this->getHighlightingDisabled()) {
+        $byte_limit = DifferentialChangesetParser::HIGHLIGHT_BYTE_LIMIT;
+        $byte_limit = phutil_format_bytes($byte_limit);
+        $messages[] = pht(
+          'This file is larger than %s, so syntax highlighting is '.
+          'disabled by default.',
+          $byte_limit);
+      }
     }
 
     return $this->formatHeaderMessages($messages);
   }
 
   private function formatHeaderMessages(array $messages) {
     if (!$messages) {
       return null;
     }
 
     foreach ($messages as $key => $message) {
       $messages[$key] = phutil_tag('li', array(), $message);
     }
 
     return phutil_tag(
       'ul',
       array(
         'class' => 'differential-meta-notice',
       ),
       $messages);
   }
 
   protected function renderPropertyChangeHeader() {
     $changeset = $this->getChangeset();
     list($old, $new) = $this->getChangesetProperties($changeset);
 
     // If we don't have any property changes, don't render this table.
     if ($old === $new) {
       return null;
     }
 
     $keys = array_keys($old + $new);
     sort($keys);
 
     $key_map = array(
       'unix:filemode' => pht('File Mode'),
       'file:dimensions' => pht('Image Dimensions'),
       'file:mimetype' => pht('MIME Type'),
       'file:size' => pht('File Size'),
     );
 
     $rows = array();
     foreach ($keys as $key) {
       $oval = idx($old, $key);
       $nval = idx($new, $key);
       if ($oval !== $nval) {
         if ($oval === null) {
           $oval = phutil_tag('em', array(), 'null');
         } else {
           $oval = phutil_escape_html_newlines($oval);
         }
 
         if ($nval === null) {
           $nval = phutil_tag('em', array(), 'null');
         } else {
           $nval = phutil_escape_html_newlines($nval);
         }
 
         $readable_key = idx($key_map, $key, $key);
 
         $row = array(
           $readable_key,
           $oval,
           $nval,
         );
         $rows[] = $row;
 
       }
     }
 
     $classes = array('', 'oval', 'nval');
     $headers = array(
       pht('Property'),
       pht('Old Value'),
       pht('New Value'),
     );
     $table = id(new AphrontTableView($rows))
       ->setHeaders($headers)
       ->setColumnClasses($classes);
     return phutil_tag(
       'div',
       array(
         'class' => 'differential-property-table',
       ),
       $table);
   }
 
   public function renderShield($message, $force = 'default') {
     $end = count($this->getOldLines());
     $reference = $this->getRenderingReference();
 
     if ($force !== 'text' &&
         $force !== 'none' &&
         $force !== 'default') {
       throw new Exception(
         pht(
           "Invalid '%s' parameter '%s'!",
           'force',
           $force));
     }
 
     $range = "0-{$end}";
     if ($force == 'text') {
       // If we're forcing text, force the whole file to be rendered.
       $range = "{$range}/0-{$end}";
     }
 
     $meta = array(
       'ref'   => $reference,
       'range' => $range,
     );
 
     $content = array();
     $content[] = $message;
     if ($force !== 'none') {
       $content[] = ' ';
       $content[] = javelin_tag(
         'a',
         array(
           'mustcapture' => true,
           'sigil'       => 'show-more',
           'class'       => 'complete',
           'href'        => '#',
           'meta'        => $meta,
         ),
         pht('Show File Contents'));
     }
 
     return $this->wrapChangeInTable(
       javelin_tag(
         'tr',
         array(
           'sigil' => 'context-target',
         ),
         phutil_tag(
           'td',
           array(
             'class' => 'differential-shield',
             'colspan' => 6,
           ),
           $content)));
   }
 
   abstract protected function renderColgroup();
 
 
   protected function wrapChangeInTable($content) {
     if (!$content) {
       return null;
     }
 
     $classes = array();
     $classes[] = 'differential-diff';
     $classes[] = 'remarkup-code';
     $classes[] = 'PhabricatorMonospaced';
     $classes[] = $this->getRendererTableClass();
 
     $sigils = array();
     $sigils[] = 'differential-diff';
     foreach ($this->getTableSigils() as $sigil) {
       $sigils[] = $sigil;
     }
 
     return javelin_tag(
       'table',
       array(
         'class' => implode(' ', $classes),
         'sigil' => implode(' ', $sigils),
       ),
       array(
         $this->renderColgroup(),
         $content,
       ));
   }
 
   protected function getTableSigils() {
     return array();
   }
 
   protected function buildInlineComment(
     PhabricatorInlineCommentInterface $comment,
     $on_right = false) {
 
     $user = $this->getUser();
     $edit = $user &&
             ($comment->getAuthorPHID() == $user->getPHID()) &&
             ($comment->isDraft())
             && $this->getShowEditAndReplyLinks();
     $allow_reply = (bool)$user && $this->getShowEditAndReplyLinks();
     $allow_done = !$comment->isDraft() && $this->getCanMarkDone();
 
     return id(new PHUIDiffInlineCommentDetailView())
       ->setUser($user)
       ->setInlineComment($comment)
       ->setIsOnRight($on_right)
       ->setHandles($this->getHandles())
       ->setMarkupEngine($this->getMarkupEngine())
       ->setEditable($edit)
       ->setAllowReply($allow_reply)
       ->setCanMarkDone($allow_done)
       ->setObjectOwnerPHID($this->getObjectOwnerPHID());
   }
 
 
   /**
    * Build links which users can click to show more context in a changeset.
    *
    * @param int Beginning of the line range to build links for.
    * @param int Length of the line range to build links for.
    * @param int Total number of lines in the changeset.
    * @return markup Rendered links.
    */
   protected function renderShowContextLinks($top, $len, $changeset_length) {
     $block_size = 20;
     $end = ($top + $len) - $block_size;
 
     // If this is a large block, such that the "top" and "bottom" ranges are
     // non-overlapping, we'll provide options to show the top, bottom or entire
     // block. For smaller blocks, we only provide an option to show the entire
     // block, since it would be silly to show the bottom 20 lines of a 25-line
     // block.
     $is_large_block = ($len > ($block_size * 2));
 
     $links = array();
 
     if ($is_large_block) {
       $is_first_block = ($top == 0);
       if ($is_first_block) {
         $text = pht('Show First %d Line(s)', $block_size);
       } else {
         $text = pht("\xE2\x96\xB2 Show %d Line(s)", $block_size);
       }
 
       $links[] = $this->renderShowContextLink(
         false,
         "{$top}-{$len}/{$top}-20",
         $text);
     }
 
     $links[] = $this->renderShowContextLink(
       true,
       "{$top}-{$len}/{$top}-{$len}",
       pht('Show All %d Line(s)', $len));
 
     if ($is_large_block) {
       $is_last_block = (($top + $len) >= $changeset_length);
       if ($is_last_block) {
         $text = pht('Show Last %d Line(s)', $block_size);
       } else {
         $text = "\xE2\x96\xBC ".pht('Show %d Line(s)', $block_size);
       }
 
       $links[] = $this->renderShowContextLink(
         false,
         "{$top}-{$len}/{$end}-20",
         $text);
     }
 
     return phutil_implode_html(" \xE2\x80\xA2 ", $links);
   }
 
 
   /**
    * Build a link that shows more context in a changeset.
    *
    * See @{method:renderShowContextLinks}.
    *
    * @param bool Does this link show all context when clicked?
    * @param string Range specification for lines to show.
    * @param string Text of the link.
    * @return markup Rendered link.
    */
   private function renderShowContextLink($is_all, $range, $text) {
     $reference = $this->getRenderingReference();
 
     return javelin_tag(
       'a',
       array(
         'href' => '#',
         'mustcapture' => true,
         'sigil' => 'show-more',
         'meta' => array(
           'type' => ($is_all ? 'all' : null),
           'range' => $range,
         ),
       ),
       $text);
   }
 
   /**
    * Build the prefixes for line IDs used to track inline comments.
    *
    * @return pair<wild, wild> Left and right prefixes.
    */
   protected function getLineIDPrefixes() {
     // These look like "C123NL45", which means the line is line 45 on the
     // "new" side of the file in changeset 123.
 
     // The "C" stands for "changeset", and is followed by a changeset ID.
 
     // "N" stands for "new" and means the comment should attach to the new file
     // when stored. "O" stands for "old" and means the comment should attach to
     // the old file. These are important because either the old or new part
     // of a file may appear on the left or right side of the diff in the
     // diff-of-diffs view.
 
     // The "L" stands for "line" and is followed by the line number.
 
     if ($this->getOldChangesetID()) {
       $left_prefix = array();
       $left_prefix[] = 'C';
       $left_prefix[] = $this->getOldChangesetID();
       $left_prefix[] = $this->getOldAttachesToNewFile() ? 'N' : 'O';
       $left_prefix[] = 'L';
       $left_prefix = implode('', $left_prefix);
     } else {
       $left_prefix = null;
     }
 
     if ($this->getNewChangesetID()) {
       $right_prefix = array();
       $right_prefix[] = 'C';
       $right_prefix[] = $this->getNewChangesetID();
       $right_prefix[] = $this->getNewAttachesToNewFile() ? 'N' : 'O';
       $right_prefix[] = 'L';
       $right_prefix = implode('', $right_prefix);
     } else {
       $right_prefix = null;
     }
 
     return array($left_prefix, $right_prefix);
   }
 
 }
diff --git a/src/applications/differential/render/DifferentialChangesetRenderer.php b/src/applications/differential/render/DifferentialChangesetRenderer.php
index e6c6f01fb7..769b1ee9f8 100644
--- a/src/applications/differential/render/DifferentialChangesetRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetRenderer.php
@@ -1,725 +1,746 @@
 <?php
 
 abstract class DifferentialChangesetRenderer extends Phobject {
 
   private $user;
   private $changeset;
   private $renderingReference;
   private $renderPropertyChangeHeader;
   private $isTopLevel;
   private $isUndershield;
   private $hunkStartLines;
   private $oldLines;
   private $newLines;
   private $oldComments;
   private $newComments;
   private $oldChangesetID;
   private $newChangesetID;
   private $oldAttachesToNewFile;
   private $newAttachesToNewFile;
   private $highlightOld = array();
   private $highlightNew = array();
   private $codeCoverage;
   private $handles;
   private $markupEngine;
   private $oldRender;
   private $newRender;
   private $originalOld;
   private $originalNew;
   private $gaps;
   private $mask;
   private $originalCharacterEncoding;
   private $showEditAndReplyLinks;
   private $canMarkDone;
   private $objectOwnerPHID;
   private $highlightingDisabled;
   private $scopeEngine = false;
   private $depthOnlyLines;
+  private $documentEngineBlocks;
 
   private $oldFile = false;
   private $newFile = false;
 
   abstract public function getRendererKey();
 
   public function setShowEditAndReplyLinks($bool) {
     $this->showEditAndReplyLinks = $bool;
     return $this;
   }
 
   public function getShowEditAndReplyLinks() {
     return $this->showEditAndReplyLinks;
   }
 
   public function setHighlightingDisabled($highlighting_disabled) {
     $this->highlightingDisabled = $highlighting_disabled;
     return $this;
   }
 
   public function getHighlightingDisabled() {
     return $this->highlightingDisabled;
   }
 
   public function setOriginalCharacterEncoding($original_character_encoding) {
     $this->originalCharacterEncoding = $original_character_encoding;
     return $this;
   }
 
   public function getOriginalCharacterEncoding() {
     return $this->originalCharacterEncoding;
   }
 
   public function setIsUndershield($is_undershield) {
     $this->isUndershield = $is_undershield;
     return $this;
   }
 
   public function getIsUndershield() {
     return $this->isUndershield;
   }
 
   public function setMask($mask) {
     $this->mask = $mask;
     return $this;
   }
   protected function getMask() {
     return $this->mask;
   }
 
   public function setGaps($gaps) {
     $this->gaps = $gaps;
     return $this;
   }
   protected function getGaps() {
     return $this->gaps;
   }
 
   public function setDepthOnlyLines(array $lines) {
     $this->depthOnlyLines = $lines;
     return $this;
   }
 
   public function getDepthOnlyLines() {
     return $this->depthOnlyLines;
   }
 
   public function attachOldFile(PhabricatorFile $old = null) {
     $this->oldFile = $old;
     return $this;
   }
 
   public function getOldFile() {
     if ($this->oldFile === false) {
       throw new PhabricatorDataNotAttachedException($this);
     }
     return $this->oldFile;
   }
 
   public function hasOldFile() {
     return (bool)$this->oldFile;
   }
 
   public function attachNewFile(PhabricatorFile $new = null) {
     $this->newFile = $new;
     return $this;
   }
 
   public function getNewFile() {
     if ($this->newFile === false) {
       throw new PhabricatorDataNotAttachedException($this);
     }
     return $this->newFile;
   }
 
   public function hasNewFile() {
     return (bool)$this->newFile;
   }
 
   public function setOriginalNew($original_new) {
     $this->originalNew = $original_new;
     return $this;
   }
   protected function getOriginalNew() {
     return $this->originalNew;
   }
 
   public function setOriginalOld($original_old) {
     $this->originalOld = $original_old;
     return $this;
   }
   protected function getOriginalOld() {
     return $this->originalOld;
   }
 
   public function setNewRender($new_render) {
     $this->newRender = $new_render;
     return $this;
   }
   protected function getNewRender() {
     return $this->newRender;
   }
 
   public function setOldRender($old_render) {
     $this->oldRender = $old_render;
     return $this;
   }
   protected function getOldRender() {
     return $this->oldRender;
   }
 
   public function setMarkupEngine(PhabricatorMarkupEngine $markup_engine) {
     $this->markupEngine = $markup_engine;
     return $this;
   }
   public function getMarkupEngine() {
     return $this->markupEngine;
   }
 
   public function setHandles(array $handles) {
     assert_instances_of($handles, 'PhabricatorObjectHandle');
     $this->handles = $handles;
     return $this;
   }
   protected function getHandles() {
     return $this->handles;
   }
 
   public function setCodeCoverage($code_coverage) {
     $this->codeCoverage = $code_coverage;
     return $this;
   }
   protected function getCodeCoverage() {
     return $this->codeCoverage;
   }
 
   public function setHighlightNew($highlight_new) {
     $this->highlightNew = $highlight_new;
     return $this;
   }
   protected function getHighlightNew() {
     return $this->highlightNew;
   }
 
   public function setHighlightOld($highlight_old) {
     $this->highlightOld = $highlight_old;
     return $this;
   }
   protected function getHighlightOld() {
     return $this->highlightOld;
   }
 
   public function setNewAttachesToNewFile($attaches) {
     $this->newAttachesToNewFile = $attaches;
     return $this;
   }
   protected function getNewAttachesToNewFile() {
     return $this->newAttachesToNewFile;
   }
 
   public function setOldAttachesToNewFile($attaches) {
     $this->oldAttachesToNewFile = $attaches;
     return $this;
   }
   protected function getOldAttachesToNewFile() {
     return $this->oldAttachesToNewFile;
   }
 
   public function setNewChangesetID($new_changeset_id) {
     $this->newChangesetID = $new_changeset_id;
     return $this;
   }
   protected function getNewChangesetID() {
     return $this->newChangesetID;
   }
 
   public function setOldChangesetID($old_changeset_id) {
     $this->oldChangesetID = $old_changeset_id;
     return $this;
   }
   protected function getOldChangesetID() {
     return $this->oldChangesetID;
   }
 
+  public function setDocumentEngineBlocks(
+    PhabricatorDocumentEngineBlocks $blocks) {
+    $this->documentEngineBlocks = $blocks;
+    return $this;
+  }
+
+  public function getDocumentEngineBlocks() {
+    return $this->documentEngineBlocks;
+  }
+
   public function setNewComments(array $new_comments) {
     foreach ($new_comments as $line_number => $comments) {
       assert_instances_of($comments, 'PhabricatorInlineCommentInterface');
     }
     $this->newComments = $new_comments;
     return $this;
   }
   protected function getNewComments() {
     return $this->newComments;
   }
 
   public function setOldComments(array $old_comments) {
     foreach ($old_comments as $line_number => $comments) {
       assert_instances_of($comments, 'PhabricatorInlineCommentInterface');
     }
     $this->oldComments = $old_comments;
     return $this;
   }
   protected function getOldComments() {
     return $this->oldComments;
   }
 
   public function setNewLines(array $new_lines) {
     $this->newLines = $new_lines;
     return $this;
   }
   protected function getNewLines() {
     return $this->newLines;
   }
 
   public function setOldLines(array $old_lines) {
     $this->oldLines = $old_lines;
     return $this;
   }
   protected function getOldLines() {
     return $this->oldLines;
   }
 
   public function setHunkStartLines(array $hunk_start_lines) {
     $this->hunkStartLines = $hunk_start_lines;
     return $this;
   }
 
   protected function getHunkStartLines() {
     return $this->hunkStartLines;
   }
 
   public function setUser(PhabricatorUser $user) {
     $this->user = $user;
     return $this;
   }
   protected function getUser() {
     return $this->user;
   }
 
   public function setChangeset(DifferentialChangeset $changeset) {
     $this->changeset = $changeset;
     return $this;
   }
   protected function getChangeset() {
     return $this->changeset;
   }
 
   public function setRenderingReference($rendering_reference) {
     $this->renderingReference = $rendering_reference;
     return $this;
   }
   protected function getRenderingReference() {
     return $this->renderingReference;
   }
 
   public function setRenderPropertyChangeHeader($should_render) {
     $this->renderPropertyChangeHeader = $should_render;
     return $this;
   }
 
   private function shouldRenderPropertyChangeHeader() {
     return $this->renderPropertyChangeHeader;
   }
 
   public function setIsTopLevel($is) {
     $this->isTopLevel = $is;
     return $this;
   }
 
   private function getIsTopLevel() {
     return $this->isTopLevel;
   }
 
   public function setCanMarkDone($can_mark_done) {
     $this->canMarkDone = $can_mark_done;
     return $this;
   }
 
   public function getCanMarkDone() {
     return $this->canMarkDone;
   }
 
   public function setObjectOwnerPHID($phid) {
     $this->objectOwnerPHID = $phid;
     return $this;
   }
 
   public function getObjectOwnerPHID() {
     return $this->objectOwnerPHID;
   }
 
   final public function renderChangesetTable($content) {
     $props = null;
     if ($this->shouldRenderPropertyChangeHeader()) {
       $props = $this->renderPropertyChangeHeader();
     }
 
     $notice = null;
     if ($this->getIsTopLevel()) {
       $force = (!$content && !$props);
+
+      // If we have DocumentEngine messages about the blocks, assume they
+      // explain why there's no content.
+      $blocks = $this->getDocumentEngineBlocks();
+      if ($blocks) {
+        if ($blocks->getMessages()) {
+          $force = false;
+        }
+      }
+
       $notice = $this->renderChangeTypeHeader($force);
     }
 
     $undershield = null;
     if ($this->getIsUndershield()) {
       $undershield = $this->renderUndershieldHeader();
     }
 
     $result = array(
       $notice,
       $props,
       $undershield,
       $content,
     );
 
     return hsprintf('%s', $result);
   }
 
   abstract public function isOneUpRenderer();
   abstract public function renderTextChange(
     $range_start,
     $range_len,
     $rows);
 
   public function renderDocumentEngineBlocks(
     PhabricatorDocumentEngineBlocks $blocks,
     $old_changeset_key,
     $new_changeset_key) {
     return null;
   }
 
   abstract protected function renderChangeTypeHeader($force);
   abstract protected function renderUndershieldHeader();
 
   protected function didRenderChangesetTableContents($contents) {
     return $contents;
   }
 
   /**
    * Render a "shield" over the diff, with a message like "This file is
    * generated and does not need to be reviewed." or "This file was completely
    * deleted." This UI element hides unimportant text so the reviewer doesn't
    * need to scroll past it.
    *
    * The shield includes a link to view the underlying content. This link
    * may force certain rendering modes when the link is clicked:
    *
    *    - `"default"`: Render the diff normally, as though it was not
    *      shielded. This is the default and appropriate if the underlying
    *      diff is a normal change, but was hidden for reasons of not being
    *      important (e.g., generated code).
    *    - `"text"`: Force the text to be shown. This is probably only relevant
    *      when a file is not changed.
    *    - `"none"`: Don't show the link (e.g., text not available).
    *
    * @param   string        Message explaining why the diff is hidden.
    * @param   string|null   Force mode, see above.
    * @return  string        Shield markup.
    */
   abstract public function renderShield($message, $force = 'default');
 
   abstract protected function renderPropertyChangeHeader();
 
   protected function buildPrimitives($range_start, $range_len) {
     $primitives = array();
 
     $hunk_starts = $this->getHunkStartLines();
 
     $mask = $this->getMask();
     $gaps = $this->getGaps();
 
     $old = $this->getOldLines();
     $new = $this->getNewLines();
     $old_render = $this->getOldRender();
     $new_render = $this->getNewRender();
     $old_comments = $this->getOldComments();
     $new_comments = $this->getNewComments();
 
     $size = count($old);
     for ($ii = $range_start; $ii < $range_start + $range_len; $ii++) {
       if (empty($mask[$ii])) {
         list($top, $len) = array_pop($gaps);
         $primitives[] = array(
           'type' => 'context',
           'top' => $top,
           'len' => $len,
         );
 
         $ii += ($len - 1);
         continue;
       }
 
       $ospec = array(
         'type' => 'old',
         'htype' => null,
         'cursor' => $ii,
         'line' => null,
         'oline' => null,
         'render' => null,
       );
 
       $nspec = array(
         'type' => 'new',
         'htype' => null,
         'cursor' => $ii,
         'line' => null,
         'oline' => null,
         'render' => null,
         'copy' => null,
         'coverage' => null,
       );
 
       if (isset($old[$ii])) {
         $ospec['line'] = (int)$old[$ii]['line'];
         $nspec['oline'] = (int)$old[$ii]['line'];
         $ospec['htype'] = $old[$ii]['type'];
         if (isset($old_render[$ii])) {
           $ospec['render'] = $old_render[$ii];
         }
       }
 
       if (isset($new[$ii])) {
         $nspec['line'] = (int)$new[$ii]['line'];
         $ospec['oline'] = (int)$new[$ii]['line'];
         $nspec['htype'] = $new[$ii]['type'];
         if (isset($new_render[$ii])) {
           $nspec['render'] = $new_render[$ii];
         }
       }
 
       if (isset($hunk_starts[$ospec['line']])) {
         $primitives[] = array(
           'type' => 'no-context',
         );
       }
 
       $primitives[] = $ospec;
       $primitives[] = $nspec;
 
       if ($ospec['line'] !== null && isset($old_comments[$ospec['line']])) {
         foreach ($old_comments[$ospec['line']] as $comment) {
           $primitives[] = array(
             'type' => 'inline',
             'comment' => $comment,
             'right' => false,
           );
         }
       }
 
       if ($nspec['line'] !== null && isset($new_comments[$nspec['line']])) {
         foreach ($new_comments[$nspec['line']] as $comment) {
           $primitives[] = array(
             'type' => 'inline',
             'comment' => $comment,
             'right' => true,
           );
         }
       }
 
       if ($hunk_starts && ($ii == $size - 1)) {
         $primitives[] = array(
           'type' => 'no-context',
         );
       }
     }
 
     if ($this->isOneUpRenderer()) {
       $primitives = $this->processPrimitivesForOneUp($primitives);
     }
 
     return $primitives;
   }
 
   private function processPrimitivesForOneUp(array $primitives) {
     // Primitives come out of buildPrimitives() in two-up format, because it
     // is the most general, flexible format. To put them into one-up format,
     // we need to filter and reorder them. In particular:
     //
     //   - We discard unchanged lines in the old file; in one-up format, we
     //     render them only once.
     //   - We group contiguous blocks of old-modified and new-modified lines, so
     //     they render in "block of old, block of new" order instead of
     //     alternating old and new lines.
 
     $out = array();
 
     $old_buf = array();
     $new_buf = array();
     foreach ($primitives as $primitive) {
       $type = $primitive['type'];
 
       if ($type == 'old') {
         if (!$primitive['htype']) {
           // This is a line which appears in both the old file and the new
           // file, or the spacer corresponding to a line added in the new file.
           // Ignore it when rendering a one-up diff.
           continue;
         }
         $old_buf[] = $primitive;
       } else if ($type == 'new') {
         if ($primitive['line'] === null) {
           // This is an empty spacer corresponding to a line removed from the
           // old file. Ignore it when rendering a one-up diff.
           continue;
         }
         if (!$primitive['htype']) {
           // If this line is the same in both versions of the file, put it in
           // the old line buffer. This makes sure inlines on old, unchanged
           // lines end up in the right place.
 
           // First, we need to flush the line buffers if they're not empty.
           if ($old_buf) {
             $out[] = $old_buf;
             $old_buf = array();
           }
           if ($new_buf) {
             $out[] = $new_buf;
             $new_buf = array();
           }
           $old_buf[] = $primitive;
         } else {
           $new_buf[] = $primitive;
         }
       } else if ($type == 'context' || $type == 'no-context') {
         $out[] = $old_buf;
         $out[] = $new_buf;
         $old_buf = array();
         $new_buf = array();
         $out[] = array($primitive);
       } else if ($type == 'inline') {
 
         // If this inline is on the left side, put it after the old lines.
         if (!$primitive['right']) {
           $out[] = $old_buf;
           $out[] = array($primitive);
           $old_buf = array();
         } else {
           $out[] = $old_buf;
           $out[] = $new_buf;
           $out[] = array($primitive);
           $old_buf = array();
           $new_buf = array();
         }
 
       } else {
         throw new Exception(pht("Unknown primitive type '%s'!", $primitive));
       }
     }
 
     $out[] = $old_buf;
     $out[] = $new_buf;
     $out = array_mergev($out);
 
     return $out;
   }
 
   protected function getChangesetProperties($changeset) {
     $old = $changeset->getOldProperties();
     $new = $changeset->getNewProperties();
 
     // When adding files, don't show the uninteresting 644 filemode change.
     if ($changeset->getChangeType() == DifferentialChangeType::TYPE_ADD &&
         $new == array('unix:filemode' => '100644')) {
       unset($new['unix:filemode']);
     }
 
     // Likewise when removing files.
     if ($changeset->getChangeType() == DifferentialChangeType::TYPE_DELETE &&
         $old == array('unix:filemode' => '100644')) {
       unset($old['unix:filemode']);
     }
 
     $metadata = $changeset->getMetadata();
 
     if ($this->hasOldFile()) {
       $file = $this->getOldFile();
       if ($file->getImageWidth()) {
         $dimensions = $file->getImageWidth().'x'.$file->getImageHeight();
         $old['file:dimensions'] = $dimensions;
       }
       $old['file:mimetype'] = $file->getMimeType();
       $old['file:size'] = phutil_format_bytes($file->getByteSize());
     } else {
       $old['file:mimetype'] = idx($metadata, 'old:file:mime-type');
       $size = idx($metadata, 'old:file:size');
       if ($size !== null) {
         $old['file:size'] = phutil_format_bytes($size);
       }
     }
 
     if ($this->hasNewFile()) {
       $file = $this->getNewFile();
       if ($file->getImageWidth()) {
         $dimensions = $file->getImageWidth().'x'.$file->getImageHeight();
         $new['file:dimensions'] = $dimensions;
       }
       $new['file:mimetype'] = $file->getMimeType();
       $new['file:size'] = phutil_format_bytes($file->getByteSize());
     } else {
       $new['file:mimetype'] = idx($metadata, 'new:file:mime-type');
       $size = idx($metadata, 'new:file:size');
       if ($size !== null) {
         $new['file:size'] = phutil_format_bytes($size);
       }
     }
 
     return array($old, $new);
   }
 
   public function renderUndoTemplates() {
     $views = array(
       'l' => id(new PHUIDiffInlineCommentUndoView())->setIsOnRight(false),
       'r' => id(new PHUIDiffInlineCommentUndoView())->setIsOnRight(true),
     );
 
     foreach ($views as $key => $view) {
       $scaffold = $this->getRowScaffoldForInline($view);
       $views[$key] = id(new PHUIDiffInlineCommentTableScaffold())
         ->addRowScaffold($scaffold);
     }
 
     return $views;
   }
 
   final protected function getScopeEngine() {
     if ($this->scopeEngine === false) {
       $hunk_starts = $this->getHunkStartLines();
 
       // If this change is missing context, don't try to identify scopes, since
       // we won't really be able to get anywhere.
       $has_multiple_hunks = (count($hunk_starts) > 1);
 
       $has_offset_hunks = false;
       if ($hunk_starts) {
         $has_offset_hunks = (head_key($hunk_starts) != 1);
       }
 
       $missing_context = ($has_multiple_hunks || $has_offset_hunks);
 
       if ($missing_context) {
         $scope_engine = null;
       } else {
         $line_map = $this->getNewLineTextMap();
         $scope_engine = id(new PhabricatorDiffScopeEngine())
           ->setLineTextMap($line_map);
       }
 
       $this->scopeEngine = $scope_engine;
     }
 
     return $this->scopeEngine;
   }
 
   private function getNewLineTextMap() {
     $new = $this->getNewLines();
 
     $text_map = array();
     foreach ($new as $new_line) {
       if (!isset($new_line['line'])) {
         continue;
       }
       $text_map[$new_line['line']] = $new_line['text'];
     }
 
     return $text_map;
   }
 
 }
diff --git a/src/applications/files/diff/PhabricatorDocumentEngineBlocks.php b/src/applications/files/diff/PhabricatorDocumentEngineBlocks.php
index fc45c204a8..0dc033d74c 100644
--- a/src/applications/files/diff/PhabricatorDocumentEngineBlocks.php
+++ b/src/applications/files/diff/PhabricatorDocumentEngineBlocks.php
@@ -1,145 +1,159 @@
 <?php
 
 final class PhabricatorDocumentEngineBlocks
   extends Phobject {
 
   private $lists = array();
+  private $messages = array();
+
+  public function addMessage($message) {
+    $this->messages[] = $message;
+    return $this;
+  }
+
+  public function getMessages() {
+    return $this->messages;
+  }
 
   public function addBlockList(PhabricatorDocumentRef $ref, array $blocks) {
     assert_instances_of($blocks, 'PhabricatorDocumentEngineBlock');
 
     $this->lists[] = array(
       'ref' => $ref,
       'blocks' => array_values($blocks),
     );
 
     return $this;
   }
 
   public function newTwoUpLayout() {
     $rows = array();
     $lists = $this->lists;
 
+    if (count($lists) != 2) {
+      return array();
+    }
+
     $specs = array();
     foreach ($this->lists as $list) {
       $specs[] = $this->newDiffSpec($list['blocks']);
     }
 
     $old_map = $specs[0]['map'];
     $new_map = $specs[1]['map'];
 
     $old_list = $specs[0]['list'];
     $new_list = $specs[1]['list'];
 
     $changeset = id(new PhabricatorDifferenceEngine())
       ->generateChangesetFromFileContent($old_list, $new_list);
 
     $hunk_parser = id(new DifferentialHunkParser())
       ->parseHunksForLineData($changeset->getHunks())
       ->reparseHunksForSpecialAttributes();
 
     $old_lines = $hunk_parser->getOldLines();
     $new_lines = $hunk_parser->getNewLines();
 
     $rows = array();
 
     $count = count($old_lines);
     for ($ii = 0; $ii < $count; $ii++) {
       $old_line = idx($old_lines, $ii);
       $new_line = idx($new_lines, $ii);
 
       if ($old_line) {
         $old_hash = rtrim($old_line['text'], "\n");
         if (!strlen($old_hash)) {
           // This can happen when one of the sources has no blocks.
           $old_block = null;
         } else {
           $old_block = array_shift($old_map[$old_hash]);
           $old_block->setDifferenceType($old_line['type']);
         }
       } else {
         $old_block = null;
       }
 
       if ($new_line) {
         $new_hash = rtrim($new_line['text'], "\n");
         if (!strlen($new_hash)) {
           $new_block = null;
         } else {
           $new_block = array_shift($new_map[$new_hash]);
           $new_block->setDifferenceType($new_line['type']);
         }
       } else {
         $new_block = null;
       }
 
       // If both lists are empty, we may generate a row which has two empty
       // blocks.
       if (!$old_block && !$new_block) {
         continue;
       }
 
       $rows[] = array(
         $old_block,
         $new_block,
       );
     }
 
     return $rows;
   }
 
   public function newOneUpLayout() {
     $rows = array();
     $lists = $this->lists;
 
     $idx = 0;
     while (true) {
       $found_any = false;
 
       $row = array();
       foreach ($lists as $list) {
         $blocks = $list['blocks'];
         $cell = idx($blocks, $idx);
 
         if ($cell !== null) {
           $found_any = true;
         }
 
         if ($cell) {
           $rows[] = $cell;
         }
       }
 
       if (!$found_any) {
         break;
       }
 
       $idx++;
     }
 
     return $rows;
   }
 
 
   private function newDiffSpec(array $blocks) {
     $map = array();
     $list = array();
 
     foreach ($blocks as $block) {
       $hash = $block->getDifferenceHash();
 
       if (!isset($map[$hash])) {
         $map[$hash] = array();
       }
       $map[$hash][] = $block;
 
       $list[] = $hash;
     }
 
     return array(
       'map' => $map,
       'list' => implode("\n", $list)."\n",
     );
   }
 
 }
diff --git a/src/applications/files/document/PhabricatorDocumentEngine.php b/src/applications/files/document/PhabricatorDocumentEngine.php
index 7e2a0d3b34..20769553fb 100644
--- a/src/applications/files/document/PhabricatorDocumentEngine.php
+++ b/src/applications/files/document/PhabricatorDocumentEngine.php
@@ -1,260 +1,260 @@
 <?php
 
 abstract class PhabricatorDocumentEngine
   extends Phobject {
 
   private $viewer;
   private $highlightedLines = array();
   private $encodingConfiguration;
   private $highlightingConfiguration;
   private $blameConfiguration = true;
 
   final public function setViewer(PhabricatorUser $viewer) {
     $this->viewer = $viewer;
     return $this;
   }
 
   final public function getViewer() {
     return $this->viewer;
   }
 
   final public function setHighlightedLines(array $highlighted_lines) {
     $this->highlightedLines = $highlighted_lines;
     return $this;
   }
 
   final public function getHighlightedLines() {
     return $this->highlightedLines;
   }
 
   final public function canRenderDocument(PhabricatorDocumentRef $ref) {
     return $this->canRenderDocumentType($ref);
   }
 
   public function canDiffDocuments(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
     return false;
   }
 
-  public function newDiffView(
+  public function newEngineBlocks(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
     throw new PhutilMethodNotImplementedException();
   }
 
   public function canConfigureEncoding(PhabricatorDocumentRef $ref) {
     return false;
   }
 
   public function canConfigureHighlighting(PhabricatorDocumentRef $ref) {
     return false;
   }
 
   public function canBlame(PhabricatorDocumentRef $ref) {
     return false;
   }
 
   final public function setEncodingConfiguration($config) {
     $this->encodingConfiguration = $config;
     return $this;
   }
 
   final public function getEncodingConfiguration() {
     return $this->encodingConfiguration;
   }
 
   final public function setHighlightingConfiguration($config) {
     $this->highlightingConfiguration = $config;
     return $this;
   }
 
   final public function getHighlightingConfiguration() {
     return $this->highlightingConfiguration;
   }
 
   final public function setBlameConfiguration($blame_configuration) {
     $this->blameConfiguration = $blame_configuration;
     return $this;
   }
 
   final public function getBlameConfiguration() {
     return $this->blameConfiguration;
   }
 
   final protected function getBlameEnabled() {
     return $this->blameConfiguration;
   }
 
   public function shouldRenderAsync(PhabricatorDocumentRef $ref) {
     return false;
   }
 
   abstract protected function canRenderDocumentType(
     PhabricatorDocumentRef $ref);
 
   final public function newDocument(PhabricatorDocumentRef $ref) {
     $can_complete = $this->canRenderCompleteDocument($ref);
     $can_partial = $this->canRenderPartialDocument($ref);
 
     if (!$can_complete && !$can_partial) {
       return $this->newMessage(
         pht(
           'This document is too large to be rendered inline. (The document '.
           'is %s bytes, the limit for this engine is %s bytes.)',
           new PhutilNumber($ref->getByteLength()),
           new PhutilNumber($this->getByteLengthLimit())));
     }
 
     return $this->newDocumentContent($ref);
   }
 
   final public function newDocumentIcon(PhabricatorDocumentRef $ref) {
     return id(new PHUIIconView())
       ->setIcon($this->getDocumentIconIcon($ref));
   }
 
   abstract protected function newDocumentContent(
     PhabricatorDocumentRef $ref);
 
   protected function getDocumentIconIcon(PhabricatorDocumentRef $ref) {
     return 'fa-file-o';
   }
 
   protected function getDocumentRenderingText(PhabricatorDocumentRef $ref) {
     return pht('Loading...');
   }
 
   final public function getDocumentEngineKey() {
     return $this->getPhobjectClassConstant('ENGINEKEY');
   }
 
   final public static function getAllEngines() {
     return id(new PhutilClassMapQuery())
       ->setAncestorClass(__CLASS__)
       ->setUniqueMethod('getDocumentEngineKey')
       ->execute();
   }
 
   final public function newSortVector(PhabricatorDocumentRef $ref) {
     $content_score = $this->getContentScore($ref);
 
     // Prefer engines which can render the entire file over engines which
     // can only render a header, and engines which can render a header over
     // engines which can't render anything.
     if ($this->canRenderCompleteDocument($ref)) {
       $limit_score = 0;
     } else if ($this->canRenderPartialDocument($ref)) {
       $limit_score = 1;
     } else {
       $limit_score = 2;
     }
 
     return id(new PhutilSortVector())
       ->addInt($limit_score)
       ->addInt(-$content_score);
   }
 
   protected function getContentScore(PhabricatorDocumentRef $ref) {
     return 2000;
   }
 
   abstract public function getViewAsLabel(PhabricatorDocumentRef $ref);
 
   public function getViewAsIconIcon(PhabricatorDocumentRef $ref) {
     $can_complete = $this->canRenderCompleteDocument($ref);
     $can_partial = $this->canRenderPartialDocument($ref);
 
     if (!$can_complete && !$can_partial) {
       return 'fa-times';
     }
 
     return $this->getDocumentIconIcon($ref);
   }
 
   public function getViewAsIconColor(PhabricatorDocumentRef $ref) {
     $can_complete = $this->canRenderCompleteDocument($ref);
 
     if (!$can_complete) {
       return 'grey';
     }
 
     return null;
   }
 
   final public static function getEnginesForRef(
     PhabricatorUser $viewer,
     PhabricatorDocumentRef $ref) {
     $engines = self::getAllEngines();
 
     foreach ($engines as $key => $engine) {
       $engine = id(clone $engine)
         ->setViewer($viewer);
 
       if (!$engine->canRenderDocument($ref)) {
         unset($engines[$key]);
         continue;
       }
 
       $engines[$key] = $engine;
     }
 
     if (!$engines) {
       throw new Exception(pht('No content engine can render this document.'));
     }
 
     $vectors = array();
     foreach ($engines as $key => $usable_engine) {
       $vectors[$key] = $usable_engine->newSortVector($ref);
     }
     $vectors = msortv($vectors, 'getSelf');
 
     return array_select_keys($engines, array_keys($vectors));
   }
 
   protected function getByteLengthLimit() {
     return (1024 * 1024 * 8);
   }
 
   protected function canRenderCompleteDocument(PhabricatorDocumentRef $ref) {
     $limit = $this->getByteLengthLimit();
     if ($limit) {
       $length = $ref->getByteLength();
       if ($length > $limit) {
         return false;
       }
     }
 
     return true;
   }
 
   protected function canRenderPartialDocument(PhabricatorDocumentRef $ref) {
     return false;
   }
 
   protected function newMessage($message) {
     return phutil_tag(
       'div',
       array(
         'class' => 'document-engine-error',
       ),
       $message);
   }
 
   final public function newLoadingContent(PhabricatorDocumentRef $ref) {
     $spinner = id(new PHUIIconView())
       ->setIcon('fa-gear')
       ->addClass('ph-spin');
 
     return phutil_tag(
       'div',
       array(
         'class' => 'document-engine-loading',
       ),
       array(
         $spinner,
         $this->getDocumentRenderingText($ref),
       ));
   }
 
 }
diff --git a/src/applications/files/document/PhabricatorImageDocumentEngine.php b/src/applications/files/document/PhabricatorImageDocumentEngine.php
index 53dcb83965..8f39df0e5b 100644
--- a/src/applications/files/document/PhabricatorImageDocumentEngine.php
+++ b/src/applications/files/document/PhabricatorImageDocumentEngine.php
@@ -1,120 +1,120 @@
 <?php
 
 final class PhabricatorImageDocumentEngine
   extends PhabricatorDocumentEngine {
 
   const ENGINEKEY = 'image';
 
   public function getViewAsLabel(PhabricatorDocumentRef $ref) {
     return pht('View as Image');
   }
 
   protected function getDocumentIconIcon(PhabricatorDocumentRef $ref) {
     return 'fa-file-image-o';
   }
 
   protected function getByteLengthLimit() {
     return (1024 * 1024 * 64);
   }
 
   public function canDiffDocuments(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
 
     // For now, we can only render a rich image diff if both documents have
     // their data stored in Files already.
 
     return ($uref->getFile() && $vref->getFile());
   }
 
-  public function newDiffView(
+  public function newEngineBlocks(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
 
     $u_blocks = $this->newDiffBlocks($uref);
     $v_blocks = $this->newDiffBlocks($vref);
 
     return id(new PhabricatorDocumentEngineBlocks())
       ->addBlockList($uref, $u_blocks)
       ->addBlockList($vref, $v_blocks);
   }
 
   private function newDiffBlocks(PhabricatorDocumentRef $ref) {
     $blocks = array();
 
     $file = $ref->getFile();
 
     $image_view = phutil_tag(
       'div',
       array(
         'class' => 'differential-image-stage',
       ),
       phutil_tag(
         'img',
         array(
           'src' => $file->getBestURI(),
         )));
 
     $hash = $file->getContentHash();
 
     $blocks[] = id(new PhabricatorDocumentEngineBlock())
       ->setBlockKey('1')
       ->addClass('diff-image-cell')
       ->setDifferenceHash($hash)
       ->setContent($image_view);
 
     return $blocks;
   }
 
   protected function canRenderDocumentType(PhabricatorDocumentRef $ref) {
     $file = $ref->getFile();
     if ($file) {
       return $file->isViewableImage();
     }
 
     $viewable_types = PhabricatorEnv::getEnvConfig('files.viewable-mime-types');
     $viewable_types = array_keys($viewable_types);
 
     $image_types = PhabricatorEnv::getEnvConfig('files.image-mime-types');
     $image_types = array_keys($image_types);
 
     return
       $ref->hasAnyMimeType($viewable_types) &&
       $ref->hasAnyMimeType($image_types);
   }
 
   protected function newDocumentContent(PhabricatorDocumentRef $ref) {
     $file = $ref->getFile();
     if ($file) {
       $source_uri = $file->getViewURI();
     } else {
       // We could use a "data:" URI here. It's not yet clear if or when we'll
       // have a ref but no backing file.
       throw new PhutilMethodNotImplementedException();
     }
 
     $image = phutil_tag(
       'img',
       array(
         'src' => $source_uri,
       ));
 
     $linked_image = phutil_tag(
       'a',
       array(
         'href' => $source_uri,
         'rel' => 'noreferrer',
       ),
       $image);
 
     $container = phutil_tag(
       'div',
       array(
         'class' => 'document-engine-image',
       ),
       $linked_image);
 
     return $container;
   }
 
 }
diff --git a/src/applications/files/document/PhabricatorJupyterDocumentEngine.php b/src/applications/files/document/PhabricatorJupyterDocumentEngine.php
index 9836797f84..17e32d0cd0 100644
--- a/src/applications/files/document/PhabricatorJupyterDocumentEngine.php
+++ b/src/applications/files/document/PhabricatorJupyterDocumentEngine.php
@@ -1,540 +1,547 @@
 <?php
 
 final class PhabricatorJupyterDocumentEngine
   extends PhabricatorDocumentEngine {
 
   const ENGINEKEY = 'jupyter';
 
   public function getViewAsLabel(PhabricatorDocumentRef $ref) {
     return pht('View as Jupyter Notebook');
   }
 
   protected function getDocumentIconIcon(PhabricatorDocumentRef $ref) {
     return 'fa-sun-o';
   }
 
   protected function getDocumentRenderingText(PhabricatorDocumentRef $ref) {
     return pht('Rendering Jupyter Notebook...');
   }
 
   public function shouldRenderAsync(PhabricatorDocumentRef $ref) {
     return true;
   }
 
   protected function getContentScore(PhabricatorDocumentRef $ref) {
     $name = $ref->getName();
 
     if (preg_match('/\\.ipynb\z/i', $name)) {
       return 2000;
     }
 
     return 500;
   }
 
   protected function canRenderDocumentType(PhabricatorDocumentRef $ref) {
     return $ref->isProbablyJSON();
   }
 
   public function canDiffDocuments(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
     return true;
   }
 
-  public function newDiffView(
+  public function newEngineBlocks(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
 
-    $u_blocks = $this->newDiffBlocks($uref);
-    $v_blocks = $this->newDiffBlocks($vref);
+    $blocks = new PhabricatorDocumentEngineBlocks();
 
-    return id(new PhabricatorDocumentEngineBlocks())
-      ->addBlockList($uref, $u_blocks)
-      ->addBlockList($vref, $v_blocks);
+    try {
+      $u_blocks = $this->newDiffBlocks($uref);
+      $v_blocks = $this->newDiffBlocks($vref);
+
+      $blocks->addBlockList($uref, $u_blocks);
+      $blocks->addBlockList($vref, $v_blocks);
+    } catch (Exception $ex) {
+      $blocks->addMessage($ex->getMessage());
+    }
+
+    return $blocks;
   }
 
   private function newDiffBlocks(PhabricatorDocumentRef $ref) {
     $viewer = $this->getViewer();
     $content = $ref->loadData();
 
     $cells = $this->newCells($content, true);
 
     $idx = 1;
     $blocks = array();
     foreach ($cells as $cell) {
       $cell_content = $this->renderJupyterCell($viewer, $cell);
 
       $notebook_table = phutil_tag(
         'table',
         array(
           'class' => 'jupyter-notebook',
         ),
         $cell_content);
 
       $container = phutil_tag(
         'div',
         array(
           'class' => 'document-engine-jupyter document-engine-diff',
         ),
         $notebook_table);
 
       // When the cell is a source code line, we can hash just the raw
       // input rather than all the cell metadata.
 
       switch (idx($cell, 'cell_type')) {
         case 'code/line':
           $hash_input = $cell['raw'];
           break;
         default:
           $hash_input = serialize($cell);
           break;
       }
 
       $hash = PhabricatorHash::digestWithNamedKey(
         $hash_input,
         'document-engine.content-digest');
 
       $blocks[] = id(new PhabricatorDocumentEngineBlock())
         ->setBlockKey($idx)
         ->setDifferenceHash($hash)
         ->setContent($container);
 
       $idx++;
     }
 
     return $blocks;
   }
 
   protected function newDocumentContent(PhabricatorDocumentRef $ref) {
     $viewer = $this->getViewer();
     $content = $ref->loadData();
 
     try {
       $cells = $this->newCells($content, false);
     } catch (Exception $ex) {
       return $this->newMessage($ex->getMessage());
     }
 
     $rows = array();
     foreach ($cells as $cell) {
       $rows[] = $this->renderJupyterCell($viewer, $cell);
     }
 
     $notebook_table = phutil_tag(
       'table',
       array(
         'class' => 'jupyter-notebook',
       ),
       $rows);
 
     $container = phutil_tag(
       'div',
       array(
         'class' => 'document-engine-jupyter',
       ),
       $notebook_table);
 
     return $container;
   }
 
   private function newCells($content, $for_diff) {
     try {
       $data = phutil_json_decode($content);
     } catch (PhutilJSONParserException $ex) {
       throw new Exception(
         pht(
           'This is not a valid JSON document and can not be rendered as '.
           'a Jupyter notebook: %s.',
           $ex->getMessage()));
     }
 
     if (!is_array($data)) {
       throw new Exception(
         pht(
           'This document does not encode a valid JSON object and can not '.
           'be rendered as a Jupyter notebook.'));
     }
 
 
     $nbformat = idx($data, 'nbformat');
     if (!strlen($nbformat)) {
       throw new Exception(
         pht(
           'This document is missing an "nbformat" field. Jupyter notebooks '.
           'must have this field.'));
     }
 
     if ($nbformat !== 4) {
       throw new Exception(
         pht(
           'This Jupyter notebook uses an unsupported version of the file '.
           'format (found version %s, expected version 4).',
           $nbformat));
     }
 
     $cells = idx($data, 'cells');
     if (!is_array($cells)) {
       throw new Exception(
         pht(
           'This Jupyter notebook does not specify a list of "cells".'));
     }
 
     if (!$cells) {
       throw new Exception(
         pht(
           'This Jupyter notebook does not specify any notebook cells.'));
     }
 
     if (!$for_diff) {
       return $cells;
     }
 
     // If we're extracting cells to build a diff view, split code cells into
     // individual lines and individual outputs. We want users to be able to
     // add inline comments to each line and each output block.
 
     $results = array();
     foreach ($cells as $cell) {
       $cell_type = idx($cell, 'cell_type');
 
       if ($cell_type !== 'code') {
         $results[] = $cell;
         continue;
       }
 
       $label = $this->newCellLabel($cell);
 
       $lines = idx($cell, 'source');
       if (!is_array($lines)) {
         $lines = array();
       }
 
       $content = $this->highlightLines($lines);
 
       $count = count($lines);
       for ($ii = 0; $ii < $count; $ii++) {
         $is_head = ($ii === 0);
         $is_last = ($ii === ($count - 1));
 
         if ($is_head) {
           $line_label = $label;
         } else {
           $line_label = null;
         }
 
         $results[] = array(
           'cell_type' => 'code/line',
           'label' => $line_label,
           'raw' => $lines[$ii],
           'display' => idx($content, $ii),
           'head' => $is_head,
           'last' => $is_last,
         );
       }
 
       $outputs = array();
       $output_list = idx($cell, 'outputs');
       if (is_array($output_list)) {
         foreach ($output_list as $output) {
           $results[] = array(
             'cell_type' => 'code/output',
             'output' => $output,
           );
         }
       }
     }
 
     return $results;
   }
 
 
   private function renderJupyterCell(
     PhabricatorUser $viewer,
     array $cell) {
 
     list($label, $content) = $this->renderJupyterCellContent($viewer, $cell);
 
     $label_cell = phutil_tag(
       'td',
       array(
         'class' => 'jupyter-label',
       ),
       $label);
 
     $classes = null;
     switch (idx($cell, 'cell_type')) {
       case 'code/line':
         $classes = 'jupyter-cell-flush';
         break;
     }
 
     $content_cell = phutil_tag(
       'td',
       array(
         'class' => $classes,
       ),
       $content);
 
     return phutil_tag(
       'tr',
       array(),
       array(
         $label_cell,
         $content_cell,
       ));
   }
 
   private function renderJupyterCellContent(
     PhabricatorUser $viewer,
     array $cell) {
 
     $cell_type = idx($cell, 'cell_type');
     switch ($cell_type) {
       case 'markdown':
         return $this->newMarkdownCell($cell);
       case 'code':
         return $this->newCodeCell($cell);
       case 'code/line':
         return $this->newCodeLineCell($cell);
       case 'code/output':
         return $this->newCodeOutputCell($cell);
     }
 
     return $this->newRawCell(id(new PhutilJSON())->encodeFormatted($cell));
   }
 
   private function newRawCell($content) {
     return array(
       null,
       phutil_tag(
         'div',
         array(
           'class' => 'jupyter-cell-raw PhabricatorMonospaced',
         ),
         $content),
     );
   }
 
   private function newMarkdownCell(array $cell) {
     $content = idx($cell, 'source');
     if (!is_array($content)) {
       $content = array();
     }
 
     $content = implode('', $content);
     $content = phutil_escape_html_newlines($content);
 
     return array(
       null,
       phutil_tag(
         'div',
         array(
           'class' => 'jupyter-cell-markdown',
         ),
         $content),
     );
   }
 
   private function newCodeCell(array $cell) {
     $label = $this->newCellLabel($cell);
 
     $content = idx($cell, 'source');
     if (!is_array($content)) {
       $content = array();
     }
 
     $content = $this->highlightLines($content);
 
     $outputs = array();
     $output_list = idx($cell, 'outputs');
     if (is_array($output_list)) {
       foreach ($output_list as $output) {
         $outputs[] = $this->newOutput($output);
       }
     }
 
     return array(
       $label,
       array(
         phutil_tag(
           'div',
           array(
             'class' =>
               'jupyter-cell-code jupyter-cell-code-block '.
               'PhabricatorMonospaced remarkup-code',
           ),
           array(
             $content,
           )),
         $outputs,
       ),
     );
   }
 
   private function newCodeLineCell(array $cell) {
     $classes = array();
     $classes[] = 'PhabricatorMonospaced';
     $classes[] = 'remarkup-code';
     $classes[] = 'jupyter-cell-code';
     $classes[] = 'jupyter-cell-code-line';
 
     if ($cell['head']) {
       $classes[] = 'jupyter-cell-code-head';
     }
 
     if ($cell['last']) {
       $classes[] = 'jupyter-cell-code-last';
     }
 
     $classes = implode(' ', $classes);
 
     return array(
       $cell['label'],
       array(
         phutil_tag(
           'div',
           array(
             'class' => $classes,
           ),
           array(
             $cell['display'],
           )),
       ),
     );
   }
 
   private function newCodeOutputCell(array $cell) {
     return array(
       null,
       $this->newOutput($cell['output']),
     );
   }
 
   private function newOutput(array $output) {
     if (!is_array($output)) {
       return pht('<Invalid Output>');
     }
 
     $classes = array(
       'jupyter-output',
       'PhabricatorMonospaced',
     );
 
     $output_name = idx($output, 'name');
     switch ($output_name) {
       case 'stderr':
         $classes[] = 'jupyter-output-stderr';
         break;
     }
 
     $output_type = idx($output, 'output_type');
     switch ($output_type) {
       case 'execute_result':
       case 'display_data':
         $data = idx($output, 'data');
 
         $image_formats = array(
           'image/png',
           'image/jpeg',
           'image/jpg',
           'image/gif',
         );
 
         foreach ($image_formats as $image_format) {
           if (!isset($data[$image_format])) {
             continue;
           }
 
           $raw_data = $data[$image_format];
           if (!is_array($raw_data)) {
             $raw_data = array($raw_data);
           }
           $raw_data = implode('', $raw_data);
 
           $content = phutil_tag(
             'img',
             array(
               'src' => 'data:'.$image_format.';base64,'.$raw_data,
             ));
 
           break 2;
         }
 
         if (isset($data['text/html'])) {
           $content = $data['text/html'];
           $classes[] = 'jupyter-output-html';
           break;
         }
 
         if (isset($data['application/javascript'])) {
           $content = $data['application/javascript'];
           $classes[] = 'jupyter-output-html';
           break;
         }
 
         if (isset($data['text/plain'])) {
           $content = $data['text/plain'];
           break;
         }
 
         break;
       case 'stream':
       default:
         $content = idx($output, 'text');
         if (!is_array($content)) {
           $content = array();
         }
         $content = implode('', $content);
         break;
     }
 
     return phutil_tag(
       'div',
       array(
         'class' => implode(' ', $classes),
       ),
       $content);
   }
 
   private function newCellLabel(array $cell) {
     $execution_count = idx($cell, 'execution_count');
     if ($execution_count) {
       $label = 'In ['.$execution_count.']:';
     } else {
       $label = null;
     }
 
     return $label;
   }
 
   private function highlightLines(array $lines) {
     $head = head($lines);
     $matches = null;
     if (preg_match('/^%%(.*)$/', $head, $matches)) {
       $restore = array_shift($lines);
       $lang = $matches[1];
     } else {
       $restore = null;
       $lang = 'py';
     }
 
     $content = PhabricatorSyntaxHighlighter::highlightWithLanguage(
       $lang,
       implode('', $lines));
     $content = phutil_split_lines($content);
 
     if ($restore !== null) {
       $language_tag = phutil_tag(
         'span',
         array(
           'class' => 'language-tag',
         ),
         $restore);
 
       array_unshift($content, $language_tag);
     }
 
     return $content;
   }
 
 }