diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -2971,6 +2971,8 @@
     'PhabricatorDeveloperPreferencesSettingsPanel' => 'applications/settings/panel/PhabricatorDeveloperPreferencesSettingsPanel.php',
     'PhabricatorDiffInlineCommentQuery' => 'infrastructure/diff/query/PhabricatorDiffInlineCommentQuery.php',
     'PhabricatorDiffPreferencesSettingsPanel' => 'applications/settings/panel/PhabricatorDiffPreferencesSettingsPanel.php',
+    'PhabricatorDiffScopeEngine' => 'infrastructure/diff/PhabricatorDiffScopeEngine.php',
+    'PhabricatorDiffScopeEngineTestCase' => 'infrastructure/diff/__tests__/PhabricatorDiffScopeEngineTestCase.php',
     'PhabricatorDifferenceEngine' => 'infrastructure/diff/PhabricatorDifferenceEngine.php',
     'PhabricatorDifferentialApplication' => 'applications/differential/application/PhabricatorDifferentialApplication.php',
     'PhabricatorDifferentialAttachCommitWorkflow' => 'applications/differential/management/PhabricatorDifferentialAttachCommitWorkflow.php',
@@ -8850,6 +8852,8 @@
     'PhabricatorDeveloperPreferencesSettingsPanel' => 'PhabricatorEditEngineSettingsPanel',
     'PhabricatorDiffInlineCommentQuery' => 'PhabricatorApplicationTransactionCommentQuery',
     'PhabricatorDiffPreferencesSettingsPanel' => 'PhabricatorEditEngineSettingsPanel',
+    'PhabricatorDiffScopeEngine' => 'Phobject',
+    'PhabricatorDiffScopeEngineTestCase' => 'PhabricatorTestCase',
     'PhabricatorDifferenceEngine' => 'Phobject',
     'PhabricatorDifferentialApplication' => 'PhabricatorApplication',
     'PhabricatorDifferentialAttachCommitWorkflow' => 'PhabricatorDifferentialManagementWorkflow',
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
@@ -1173,7 +1173,7 @@
     }
     $range_len = min($range_len, $rows - $range_start);
 
-    list($gaps, $mask, $depths) = $this->calculateGapsMaskAndDepths(
+    list($gaps, $mask) = $this->calculateGapsAndMask(
       $mask_force,
       $feedback_mask,
       $range_start,
@@ -1181,8 +1181,7 @@
 
     $renderer
       ->setGaps($gaps)
-      ->setMask($mask)
-      ->setDepths($depths);
+      ->setMask($mask);
 
     $html = $renderer->renderTextChange(
       $range_start,
@@ -1208,15 +1207,9 @@
    * "show more"). The $mask returned is a sparsely populated dictionary
    * of $visible_line_number => true.
    *
-   * Depths - compute how indented any given line is. The $depths returned
-   * is a sparsely populated dictionary of $visible_line_number => $depth.
-   *
-   * This function also has the side effect of modifying member variable
-   * new such that tabs are normalized to spaces for each line of the diff.
-   *
-   * @return array($gaps, $mask, $depths)
+   * @return array($gaps, $mask)
    */
-  private function calculateGapsMaskAndDepths(
+  private function calculateGapsAndMask(
     $mask_force,
     $feedback_mask,
     $range_start,
@@ -1224,7 +1217,6 @@
 
     $lines_context = $this->getLinesOfContext();
 
-    // Calculate gaps and mask first
     $gaps = array();
     $gap_start = 0;
     $in_gap = false;
@@ -1253,38 +1245,7 @@
     $gaps = array_reverse($gaps);
     $mask = $base_mask;
 
-    // Time to calculate depth.
-    // We need to go backwards to properly indent whitespace in this code:
-    //
-    //   0: class C {
-    //   1:
-    //   1:   function f() {
-    //   2:
-    //   2:     return;
-    //   1:
-    //   1:   }
-    //   0:
-    //   0: }
-    //
-    $depths = array();
-    $last_depth = 0;
-    $range_end = $range_start + $range_len;
-    if (!isset($this->new[$range_end])) {
-      $range_end--;
-    }
-    for ($ii = $range_end; $ii >= $range_start; $ii--) {
-      // We need to expand tabs to process mixed indenting and to round
-      // correctly later.
-      $line = str_replace("\t", '  ', $this->new[$ii]['text']);
-      $trimmed = ltrim($line);
-      if ($trimmed != '') {
-        // We round down to flatten "/**" and " *".
-        $last_depth = floor((strlen($line) - strlen($trimmed)) / 2);
-      }
-      $depths[$ii] = $last_depth;
-    }
-
-    return array($gaps, $mask, $depths);
+    return array($gaps, $mask);
   }
 
   /**
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
@@ -28,12 +28,12 @@
   private $originalNew;
   private $gaps;
   private $mask;
-  private $depths;
   private $originalCharacterEncoding;
   private $showEditAndReplyLinks;
   private $canMarkDone;
   private $objectOwnerPHID;
   private $highlightingDisabled;
+  private $scopeEngine;
 
   private $oldFile = false;
   private $newFile = false;
@@ -76,14 +76,6 @@
     return $this->isUndershield;
   }
 
-  public function setDepths($depths) {
-    $this->depths = $depths;
-    return $this;
-  }
-  protected function getDepths() {
-    return $this->depths;
-  }
-
   public function setMask($mask) {
     $this->mask = $mask;
     return $this;
@@ -678,4 +670,32 @@
     return $views;
   }
 
+
+  final protected function getScopeEngine() {
+    if (!$this->scopeEngine) {
+      $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/differential/render/DifferentialChangesetTwoUpRenderer.php b/src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php
--- a/src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php
@@ -3,6 +3,8 @@
 final class DifferentialChangesetTwoUpRenderer
   extends DifferentialChangesetHTMLRenderer {
 
+  private $newOffsetMap;
+
   public function isOneUpRenderer() {
     return false;
   }
@@ -66,9 +68,12 @@
     $new_render = $this->getNewRender();
     $original_left = $this->getOriginalOld();
     $original_right = $this->getOriginalNew();
-    $depths = $this->getDepths();
     $mask = $this->getMask();
 
+    $scope_engine = $this->getScopeEngine();
+
+    $offset_map = null;
+
     for ($ii = $range_start; $ii < $range_start + $range_len; $ii++) {
       if (empty($mask[$ii])) {
         // If we aren't going to show this line, we've just entered a gap.
@@ -87,16 +92,19 @@
           $is_last_block = true;
         }
 
-        $context = null;
+        $context_text = null;
         $context_line = null;
-        if (!$is_last_block && $depths[$ii + $len]) {
-          for ($l = $ii + $len - 1; $l >= $ii; $l--) {
-            $line = $new_lines[$l]['text'];
-            if ($depths[$l] < $depths[$ii + $len] && trim($line) != '') {
-              $context = $new_render[$l];
-              $context_line = $new_lines[$l]['line'];
-              break;
+        if (!$is_last_block) {
+          $target_line = $new_lines[$ii + $len]['line'];
+          $context_line = $scope_engine->getScopeStart($target_line);
+          if ($context_line !== null) {
+            // The scope engine returns a line number in the file. We need
+            // to map that back to a display offset in the diff.
+            if (!$offset_map) {
+              $offset_map = $this->getNewLineToOffsetMap();
             }
+            $offset = $offset_map[$context_line];
+            $context_text = $new_render[$offset];
           }
         }
 
@@ -126,7 +134,7 @@
                 'class' => 'show-context',
               ),
               // TODO: [HTML] Escaping model here isn't ideal.
-              phutil_safe_html($context)),
+              phutil_safe_html($context_text)),
           ));
 
         $html[] = $container;
@@ -386,4 +394,22 @@
       ->addInlineView($view);
   }
 
+  private function getNewLineToOffsetMap() {
+    if ($this->newOffsetMap === null) {
+      $new = $this->getNewLines();
+
+      $map = array();
+      foreach ($new as $offset => $new_line) {
+        if ($new_line['line'] === null) {
+          continue;
+        }
+        $map[$new_line['line']] = $offset;
+      }
+
+      $this->newOffsetMap = $map;
+    }
+
+    return $this->newOffsetMap;
+  }
+
 }
diff --git a/src/infrastructure/diff/PhabricatorDiffScopeEngine.php b/src/infrastructure/diff/PhabricatorDiffScopeEngine.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/diff/PhabricatorDiffScopeEngine.php
@@ -0,0 +1,156 @@
+<?php
+
+final class PhabricatorDiffScopeEngine
+  extends Phobject {
+
+  private $lineTextMap;
+  private $lineDepthMap;
+
+  public function setLineTextMap(array $map) {
+    if (array_key_exists(0, $map)) {
+      throw new Exception(
+        pht('ScopeEngine text map must be a 1-based map of lines.'));
+    }
+
+    $expect = 1;
+    foreach ($map as $key => $value) {
+      if ($key === $expect) {
+        $expect++;
+        continue;
+      }
+
+      throw new Exception(
+        pht(
+          'ScopeEngine text map must be a contiguous map of '.
+          'lines, but is not: found key "%s" where key "%s" was expected.',
+          $key,
+          $expect));
+    }
+
+    $this->lineTextMap = $map;
+
+    return $this;
+  }
+
+  public function getLineTextMap() {
+    if ($this->lineTextMap === null) {
+      throw new PhutilInvalidStateException('setLineTextMap');
+    }
+    return $this->lineTextMap;
+  }
+
+  public function getScopeStart($line) {
+    $text_map = $this->getLineTextMap();
+    $depth_map = $this->getLineDepthMap();
+    $length = count($text_map);
+
+    // Figure out the effective depth of the line we're getting scope for.
+    // If the line is just whitespace, it may have no depth on its own. In
+    // this case, we look for the next line.
+    $line_depth = null;
+    for ($ii = $line; $ii <= $length; $ii++) {
+      if ($depth_map[$ii] !== null) {
+        $line_depth = $depth_map[$ii];
+        break;
+      }
+    }
+
+    // If we can't find a line depth for the target line, just bail.
+    if ($line_depth === null) {
+      return null;
+    }
+
+    // Limit the maximum number of lines we'll examine. If a user has a
+    // million-line diff of nonsense, scanning the whole thing is a waste
+    // of time.
+    $search_range = 1000;
+    $search_until = max(0, $ii - $search_range);
+
+    for ($ii = $line - 1; $ii > $search_until; $ii--) {
+      $line_text = $text_map[$ii];
+
+      // This line is in missing context: the diff was diffed with partial
+      // context, and we ran out of context before finding a good scope line.
+      // Bail out, we don't want to jump across missing context blocks.
+      if ($line_text === null) {
+        return null;
+      }
+
+      $depth = $depth_map[$ii];
+
+      // This line is all whitespace. This isn't a possible match.
+      if ($depth === null) {
+        continue;
+      }
+
+      // The depth is the same as (or greater than) the depth we started with,
+      // so this isn't a possible match.
+      if ($depth >= $line_depth) {
+        continue;
+      }
+
+      // Reject lines which begin with "}" or "{". These lines are probably
+      // never good matches.
+      if (preg_match('/^\s*[{}]/i', $line_text)) {
+        continue;
+      }
+
+      return $ii;
+    }
+
+    return null;
+  }
+
+  private function getLineDepthMap() {
+    if (!$this->lineDepthMap) {
+      $this->lineDepthMap = $this->newLineDepthMap();
+    }
+
+    return $this->lineDepthMap;
+  }
+
+  private function newLineDepthMap() {
+    $text_map = $this->getLineTextMap();
+
+    // TODO: This should be configurable once we handle tab widths better.
+    $tab_width = 2;
+
+    $depth_map = array();
+    foreach ($text_map as $line_number => $line_text) {
+      if ($line_text === null) {
+        $depth_map[$line_number] = null;
+        continue;
+      }
+
+      $len = strlen($line_text);
+
+      // If the line has no actual text, don't assign it a depth.
+      if (!$len || !strlen(trim($line_text))) {
+        $depth_map[$line_number] = null;
+        continue;
+      }
+
+      $count = 0;
+      for ($ii = 0; $ii < $len; $ii++) {
+        $c = $line_text[$ii];
+        if ($c == ' ') {
+          $count++;
+        } else if ($c == "\t") {
+          $count += $tab_width;
+        } else {
+          break;
+        }
+      }
+
+      // Round down to cheat our way through the " *" parts of docblock
+      // comments. This is generally a reasonble heuristic because odd tab
+      // widths are exceptionally rare.
+      $depth = ($count >> 1);
+
+      $depth_map[$line_number] = $depth;
+    }
+
+    return $depth_map;
+  }
+
+}
diff --git a/src/infrastructure/diff/__tests__/PhabricatorDiffScopeEngineTestCase.php b/src/infrastructure/diff/__tests__/PhabricatorDiffScopeEngineTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/diff/__tests__/PhabricatorDiffScopeEngineTestCase.php
@@ -0,0 +1,51 @@
+<?php
+
+final class PhabricatorDiffScopeEngineTestCase
+  extends PhabricatorTestCase {
+
+  private $engines = array();
+
+  public function testScopeEngine() {
+    $this->assertScopeStart('zebra.c', 4, 2);
+  }
+
+  private function assertScopeStart($file, $line, $expect) {
+    $engine = $this->getScopeTestEngine($file);
+
+    $actual = $engine->getScopeStart($line);
+    $this->assertEqual(
+      $expect,
+      $actual,
+      pht(
+        'Expect scope for line %s to start on line %s (actual: %s) in "%s".',
+        $line,
+        $expect,
+        $actual,
+        $file));
+  }
+
+  private function getScopeTestEngine($file) {
+    if (!isset($this->engines[$file])) {
+      $this->engines[$file] = $this->newScopeTestEngine($file);
+    }
+
+    return $this->engines[$file];
+  }
+
+  private function newScopeTestEngine($file) {
+    $path = dirname(__FILE__).'/data/'.$file;
+    $data = Filesystem::readFile($path);
+
+    $lines = phutil_split_lines($data);
+    $map = array();
+    foreach ($lines as $key => $line) {
+      $map[$key + 1] = $line;
+    }
+
+    $engine = id(new PhabricatorDiffScopeEngine())
+      ->setLineTextMap($map);
+
+    return $engine;
+  }
+
+}
diff --git a/src/infrastructure/diff/__tests__/data/zebra.c b/src/infrastructure/diff/__tests__/data/zebra.c
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/diff/__tests__/data/zebra.c
@@ -0,0 +1,5 @@
+void
+ZebraTamer::TameAZebra(nsPoint where, const nsRect& zone, nsAtom* material)
+{
+  zebra.tame = true;
+}