diff --git a/resources/celerity/map.php b/resources/celerity/map.php
--- a/resources/celerity/map.php
+++ b/resources/celerity/map.php
@@ -363,7 +363,7 @@
     'rsrc/js/application/dashboard/behavior-dashboard-move-panels.js' => '82439934',
     'rsrc/js/application/dashboard/behavior-dashboard-query-panel-select.js' => '453c5375',
     'rsrc/js/application/dashboard/behavior-dashboard-tab-panel.js' => 'd4eecc63',
-    'rsrc/js/application/differential/ChangesetViewManager.js' => '88be0133',
+    'rsrc/js/application/differential/ChangesetViewManager.js' => '58562350',
     'rsrc/js/application/differential/DifferentialInlineCommentEditor.js' => 'f2431bc1',
     'rsrc/js/application/differential/behavior-add-reviewers-and-ccs.js' => 'e10f8e18',
     'rsrc/js/application/differential/behavior-comment-jump.js' => '4fdb476d',
@@ -510,7 +510,7 @@
     'aphront-two-column-view-css' => '16ab3ad2',
     'aphront-typeahead-control-css' => '0e403212',
     'auth-css' => '1e655982',
-    'changeset-view-manager' => '88be0133',
+    'changeset-view-manager' => '58562350',
     'config-options-css' => '7fedf08b',
     'config-welcome-css' => '6abd79be',
     'conpherence-durable-column-view' => 'a27580c5',
@@ -1202,6 +1202,16 @@
       'javelin-vector',
       'javelin-dom',
     ),
+    58562350 => array(
+      'javelin-dom',
+      'javelin-util',
+      'javelin-stratcom',
+      'javelin-install',
+      'javelin-workflow',
+      'javelin-router',
+      'javelin-behavior-device',
+      'javelin-vector',
+    ),
     '59b251eb' => array(
       'javelin-behavior',
       'javelin-stratcom',
@@ -1492,16 +1502,6 @@
       'javelin-stratcom',
       'javelin-dom',
     ),
-    '88be0133' => array(
-      'javelin-dom',
-      'javelin-util',
-      'javelin-stratcom',
-      'javelin-install',
-      'javelin-workflow',
-      'javelin-router',
-      'javelin-behavior-device',
-      'javelin-vector',
-    ),
     '88f0c5b3' => array(
       'javelin-behavior',
       'javelin-dom',
diff --git a/src/applications/differential/parser/DifferentialChangesetParser.php b/src/applications/differential/parser/DifferentialChangesetParser.php
--- a/src/applications/differential/parser/DifferentialChangesetParser.php
+++ b/src/applications/differential/parser/DifferentialChangesetParser.php
@@ -2,6 +2,8 @@
 
 final class DifferentialChangesetParser {
 
+  const HIGHLIGHT_BYTE_LIMIT = 262144;
+
   protected $visible      = array();
   protected $new          = array();
   protected $old          = array();
@@ -36,6 +38,7 @@
   private $isSubparser;
 
   private $isTopLevel;
+
   private $coverage;
   private $markupEngine;
   private $highlightErrors;
@@ -43,6 +46,7 @@
   private $renderer;
   private $characterEncoding;
   private $highlightAs;
+  private $highlightingDisabled;
   private $showEditAndReplyLinks = true;
   private $canMarkDone;
 
@@ -69,6 +73,7 @@
     $this->showEditAndReplyLinks = $bool;
     return $this;
   }
+
   public function getShowEditAndReplyLinks() {
     return $this->showEditAndReplyLinks;
   }
@@ -416,6 +421,7 @@
       'hunkStartLines',
       'cacheVersion',
       'cacheHost',
+      'highlightingDisabled',
     );
   }
 
@@ -537,6 +543,12 @@
     if (!$language) {
       $language = $this->highlightEngine->getLanguageFromFilename(
         $this->filename);
+
+      if (($language != 'txt') &&
+          (strlen($corpus) > self::HIGHLIGHT_BYTE_LIMIT)) {
+        $this->highlightingDisabled = true;
+        $language = 'txt';
+      }
     }
 
     return $this->highlightEngine->getHighlightFuture(
@@ -830,7 +842,8 @@
       ->setNewLines($this->new)
       ->setOriginalCharacterEncoding($encoding)
       ->setShowEditAndReplyLinks($this->getShowEditAndReplyLinks())
-      ->setCanMarkDone($this->getCanMarkDone());
+      ->setCanMarkDone($this->getCanMarkDone())
+      ->setHighlightingDisabled($this->highlightingDisabled);
 
     $shield = null;
     if ($this->isTopLevel && !$this->comments) {
@@ -894,6 +907,14 @@
       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();
diff --git a/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php b/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
--- a/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetHTMLRenderer.php
@@ -242,6 +242,16 @@
         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) &&
@@ -261,6 +271,17 @@
       }
     }
 
+    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));
+    }
+
+    return $this->formatHeaderMessages($messages);
+  }
+
+  private function formatHeaderMessages(array $messages) {
     if (!$messages) {
       return null;
     }
diff --git a/src/applications/differential/render/DifferentialChangesetRenderer.php b/src/applications/differential/render/DifferentialChangesetRenderer.php
--- a/src/applications/differential/render/DifferentialChangesetRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetRenderer.php
@@ -7,6 +7,7 @@
   private $renderingReference;
   private $renderPropertyChangeHeader;
   private $isTopLevel;
+  private $isUndershield;
   private $hunkStartLines;
   private $oldLines;
   private $newLines;
@@ -31,6 +32,7 @@
   private $originalCharacterEncoding;
   private $showEditAndReplyLinks;
   private $canMarkDone;
+  private $highlightingDisabled;
 
   private $oldFile = false;
   private $newFile = false;
@@ -41,10 +43,20 @@
     $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;
@@ -54,6 +66,15 @@
     return $this->originalCharacterEncoding;
   }
 
+  public function setIsUndershield($is_undershield) {
+    $this->isUndershield = $is_undershield;
+    return $this;
+  }
+
+  public function getIsUndershield() {
+    return $this->isUndershield;
+  }
+
   public function setDepths($depths) {
     $this->depths = $depths;
     return $this;
@@ -325,7 +346,12 @@
       $notice = $this->renderChangeTypeHeader($force);
     }
 
-    $result = $notice.$props.$content;
+    $undershield = null;
+    if ($this->getIsUndershield()) {
+      $undershield = $this->renderUndershieldHeader();
+    }
+
+    $result = $notice.$props.$undershield.$content;
 
     // TODO: Let the user customize their tab width / display style.
     // TODO: We should possibly post-process "\r" as well.
@@ -347,6 +373,7 @@
     $vs = 0);
 
   abstract protected function renderChangeTypeHeader($force);
+  abstract protected function renderUndershieldHeader();
 
   protected function didRenderChangesetTableContents($contents) {
     return $contents;
diff --git a/src/applications/differential/render/DifferentialChangesetTestRenderer.php b/src/applications/differential/render/DifferentialChangesetTestRenderer.php
--- a/src/applications/differential/render/DifferentialChangesetTestRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetTestRenderer.php
@@ -20,6 +20,10 @@
            "{$away}\n";
   }
 
+  protected function renderUndershieldHeader() {
+    return null;
+  }
+
   public function renderShield($message, $force = 'default') {
     return "SHIELD ({$force}) {$message}\n";
   }
diff --git a/webroot/rsrc/js/application/differential/ChangesetViewManager.js b/webroot/rsrc/js/application/differential/ChangesetViewManager.js
--- a/webroot/rsrc/js/application/differential/ChangesetViewManager.js
+++ b/webroot/rsrc/js/application/differential/ChangesetViewManager.js
@@ -190,7 +190,19 @@
      * Receive a response to a context request.
      */
     _oncontext: function(target, response) {
-      var table = JX.$H(response.changeset).getNode();
+      // TODO: This should be better structured.
+      // If the response comes back with several top-level nodes, the last one
+      // is the actual context; the others are headers. Add any headers first,
+      // then copy the new rows into the document.
+      var markup = JX.$H(response.changeset).getFragment();
+      var len = markup.childNodes.length;
+      var diff = JX.DOM.findAbove(target, 'table', 'differential-diff');
+
+      for (var ii = 0; ii < len - 1; ii++) {
+        diff.parentNode.insertBefore(markup.firstChild, diff);
+      }
+
+      var table = markup.firstChild;
       var root = target.parentNode;
       this._moveRows(table, root, target);
       root.removeChild(target);