diff --git a/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor.js b/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor.js
index 01aaa9c..3de3ae4 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor.js
@@ -278,7 +278,7 @@
         }
 
         for (i = 0;
-            i < splice.removed && splicee.removed.length;
+            i < splice.removed && splice.removed.length;
             i++) {
           this.unlisten(splice.removed[i], 'render', 'handleDiffUpdate');
         }
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder-side-by-side.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder-side-by-side.js
index bf93112..7e8779f 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder-side-by-side.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder-side-by-side.js
@@ -20,7 +20,7 @@
   GrDiffBuilderSideBySide.prototype = Object.create(GrDiffBuilder.prototype);
   GrDiffBuilderSideBySide.prototype.constructor = GrDiffBuilderSideBySide;
 
-  GrDiffBuilderSideBySide.prototype.emitGroup = function(group,
+  GrDiffBuilderSideBySide.prototype.buildSectionElement = function(group,
       opt_beforeSection) {
     var sectionEl = this._createElement('tbody', 'section');
     sectionEl.classList.add(group.type);
@@ -29,7 +29,7 @@
       sectionEl.appendChild(this._createRow(sectionEl, pairs[i].left,
           pairs[i].right));
     }
-    this._outputEl.insertBefore(sectionEl, opt_beforeSection);
+    return sectionEl;
   };
 
   GrDiffBuilderSideBySide.prototype._createRow = function(section, leftLine,
@@ -48,13 +48,14 @@
 
   GrDiffBuilderSideBySide.prototype._appendPair = function(section, row, line,
       lineNumber, side) {
-    row.appendChild(this._createLineEl(line, lineNumber, line.type, side));
+    var lineEl = this._createLineEl(line, lineNumber, line.type, side);
+    lineEl.classList.add(side);
+    row.appendChild(lineEl);
     var action = this._createContextControl(section, line);
     if (action) {
       row.appendChild(action);
     } else {
       var textEl = this._createTextEl(line);
-      textEl.classList.add(side);
       var threadEl = this._commentThreadForLine(line, side);
       if (threadEl) {
         textEl.appendChild(threadEl);
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder-unified.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder-unified.js
index 86340bd..2f1aac6 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder-unified.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder-unified.js
@@ -20,23 +20,26 @@
   GrDiffBuilderUnified.prototype = Object.create(GrDiffBuilder.prototype);
   GrDiffBuilderUnified.prototype.constructor = GrDiffBuilderUnified;
 
-  GrDiffBuilderUnified.prototype.emitGroup = function(group,
-      opt_beforeSection) {
+  GrDiffBuilderUnified.prototype.buildSectionElement = function(group) {
     var sectionEl = this._createElement('tbody', 'section');
     sectionEl.classList.add(group.type);
 
     for (var i = 0; i < group.lines.length; ++i) {
       sectionEl.appendChild(this._createRow(sectionEl, group.lines[i]));
     }
-    this._outputEl.insertBefore(sectionEl, opt_beforeSection);
+    return sectionEl;
   };
 
   GrDiffBuilderUnified.prototype._createRow = function(section, line) {
     var row = this._createElement('tr', line.type);
-    row.appendChild(this._createLineEl(line, line.beforeNumber,
-        GrDiffLine.Type.REMOVE));
-    row.appendChild(this._createLineEl(line, line.afterNumber,
-        GrDiffLine.Type.ADD));
+    var lineEl = this._createLineEl(line, line.beforeNumber,
+        GrDiffLine.Type.REMOVE);
+    lineEl.classList.add('left');
+    row.appendChild(lineEl);
+    lineEl = this._createLineEl(line, line.afterNumber,
+        GrDiffLine.Type.ADD);
+    lineEl.classList.add('right');
+    row.appendChild(lineEl);
     row.classList.add('diff-row', 'unified');
 
     var action = this._createContextControl(section, line);
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder.js
index f55037c..d2a7c25 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder.js
@@ -57,8 +57,51 @@
     }
   };
 
+  GrDiffBuilder.prototype.buildSectionElement = function(
+      group, opt_beforeSection) {
+    throw Error('Subclasses must implement buildGroupElement');
+  };
+
   GrDiffBuilder.prototype.emitGroup = function(group, opt_beforeSection) {
-    throw Error('Subclasses must implement emitGroup');
+    var element = this.buildSectionElement(group);
+    this._outputEl.insertBefore(element, opt_beforeSection);
+    group.element = element;
+  };
+
+  GrDiffBuilder.prototype.renderSection = function(element) {
+    for (var i = 0; i < this._groups.length; i++) {
+      var group = this._groups[i];
+      if (group.element === element) {
+        var newElement = this.buildSectionElement(group);
+        group.element.parentElement.replaceChild(newElement, group.element);
+        group.element = newElement;
+        break;
+      }
+    }
+  };
+
+  GrDiffBuilder.prototype.getSectionsByLineRange = function(
+      startLine, endLine, opt_side) {
+    var sections = [];
+    for (var i = 0; i < this._groups.length; i++) {
+      var group = this._groups[i];
+      if (group.lines.length === 0) {
+        continue;
+      }
+      var groupStartLine;
+      var groupEndLine;
+      if (opt_side === GrDiffBuilder.Side.LEFT) {
+        groupStartLine = group.lines[0].beforeNumber;
+        groupEndLine = group.lines[group.lines.length - 1].beforeNumber;
+      } else if (opt_side === GrDiffBuilder.Side.RIGHT) {
+        groupStartLine = group.lines[0].afterNumber;
+        groupEndLine = group.lines[group.lines.length - 1].afterNumber;
+      }
+      if (startLine <= groupEndLine && endLine >= groupStartLine) {
+        sections.push(group.element);
+      }
+    }
+    return sections;
   };
 
   GrDiffBuilder.prototype._processContent = function(content, groups, context) {
@@ -183,6 +226,7 @@
           currentChunk.ab.push(chunk[j]);
         }
       }
+      // != instead of !== because we want to cover both undefined and null.
       if (currentChunk.ab != null && currentChunk.ab.length > 0) {
         result.push(currentChunk);
       }
@@ -261,7 +305,8 @@
     }
 
     var ctxLine = new GrDiffLine(GrDiffLine.Type.CONTEXT_CONTROL);
-    ctxLine.contextLines = hiddenLines;
+    ctxLine.contextGroup =
+        new GrDiffGroup(GrDiffGroup.Type.BOTH, hiddenLines);
     groups.push(new GrDiffGroup(GrDiffGroup.Type.CONTEXT_CONTROL,
         [ctxLine]));
 
@@ -311,13 +356,14 @@
   };
 
   GrDiffBuilder.prototype._createContextControl = function(section, line) {
-    if (!line.contextLines.length) {
+    if (!line.contextGroup || !line.contextGroup.lines.length) {
       return null;
     }
+    var contextLines = line.contextGroup.lines;
     var td = this._createElement('td');
     var button = this._createElement('gr-button', 'showContext');
     button.setAttribute('link', true);
-    var commonLines = line.contextLines.length;
+    var commonLines = contextLines.length;
     var text = 'Show ' + commonLines + ' common line';
     if (commonLines > 1) {
       text += 's';
@@ -326,7 +372,7 @@
     button.textContent = text;
     button.addEventListener('tap', function(e) {
       e.detail = {
-        group: new GrDiffGroup(GrDiffGroup.Type.BOTH, line.contextLines),
+        group: line.contextGroup,
         section: section,
       };
       // Let it bubble up the DOM tree.
@@ -383,7 +429,7 @@
     }
 
     var patchNum = this._comments.meta.patchRange.patchNum;
-    var side = 'REVISION';
+    var side = comments[0].side || 'REVISION';
     if (line.type === GrDiffLine.Type.REMOVE ||
         opt_side === GrDiffBuilder.Side.LEFT) {
       if (this._comments.meta.patchRange.basePatchNum === 'PARENT') {
@@ -413,7 +459,7 @@
     } else if (line.type === GrDiffLine.Type.CONTEXT_CONTROL) {
       td.classList.add('contextLineNum');
       td.setAttribute('data-value', '@@');
-    } else if (line.type === GrDiffLine.Type.BOTH || line.type == type) {
+    } else if (line.type === GrDiffLine.Type.BOTH || line.type === type) {
       td.classList.add('lineNum');
       td.setAttribute('data-value', number);
     }
@@ -476,18 +522,18 @@
 
     // Tags don't count as characters
     while (index < html.length &&
-           html.charCodeAt(index) == GrDiffBuilder.LESS_THAN_CODE) {
+           html.charCodeAt(index) === GrDiffBuilder.LESS_THAN_CODE) {
       while (index < html.length &&
-             html.charCodeAt(index) != GrDiffBuilder.GREATER_THAN_CODE) {
+             html.charCodeAt(index) !== GrDiffBuilder.GREATER_THAN_CODE) {
         index++;
       }
       index++;  // skip the ">" itself
     }
     // An HTML entity (e.g., &lt;) counts as one character.
     if (index < html.length &&
-        html.charCodeAt(index) == GrDiffBuilder.AMPERSAND_CODE) {
+        html.charCodeAt(index) === GrDiffBuilder.AMPERSAND_CODE) {
       while (index < html.length &&
-             html.charCodeAt(index) != GrDiffBuilder.SEMICOLON_CODE) {
+             html.charCodeAt(index) !== GrDiffBuilder.SEMICOLON_CODE) {
         index++;
       }
     }
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder_test.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder_test.html
index 22b9072..4392de0 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-builder_test.html
@@ -144,8 +144,9 @@
       assert.equal(groups[0].lines[0].afterNumber, GrDiffLine.FILE);
 
       assert.equal(groups[1].type, GrDiffGroup.Type.CONTEXT_CONTROL);
-      assert.equal(groups[1].lines[0].contextLines.length, 90);
-      groups[1].lines[0].contextLines.forEach(function(l) {
+      assert.instanceOf(groups[1].lines[0].contextGroup, GrDiffGroup);
+      assert.equal(groups[1].lines[0].contextGroup.lines.length, 90);
+      groups[1].lines[0].contextGroup.lines.forEach(function(l) {
         assert.equal(l.text, content[0].ab[0]);
       });
 
@@ -179,8 +180,9 @@
       });
 
       assert.equal(groups[7].type, GrDiffGroup.Type.CONTEXT_CONTROL);
-      assert.equal(groups[7].lines[0].contextLines.length, 90);
-      groups[7].lines[0].contextLines.forEach(function(l) {
+      assert.instanceOf(groups[7].lines[0].contextGroup, GrDiffGroup);
+      assert.equal(groups[7].lines[0].contextGroup.lines.length, 90);
+      groups[7].lines[0].contextGroup.lines.forEach(function(l) {
         assert.equal(l.text, content[4].ab[0]);
       });
 
@@ -215,8 +217,9 @@
       });
 
       assert.equal(groups[3].type, GrDiffGroup.Type.CONTEXT_CONTROL);
-      assert.equal(groups[3].lines[0].contextLines.length, 30);
-      groups[3].lines[0].contextLines.forEach(function(l) {
+      assert.instanceOf(groups[3].lines[0].contextGroup, GrDiffGroup);
+      assert.equal(groups[3].lines[0].contextGroup.lines.length, 30);
+      groups[3].lines[0].contextGroup.lines.forEach(function(l) {
         assert.equal(l.text, content[1].ab[0]);
       });
 
@@ -517,5 +520,54 @@
         }
       ]);
     });
+
+    suite('rendering', function() {
+      var content;
+      var outputEl;
+
+      setup(function() {
+        var prefs = {
+          line_length: 10,
+          show_tabs: true,
+          tab_size: 4,
+          context: -1
+        };
+        content = [
+          {ab: []},
+          {a: ['all work and no play make andybons a dull boy']},
+          {ab: []},
+          {b: ['elgoog elgoog elgoog']},
+          {ab: []},
+        ];
+        outputEl = document.createElement('out');
+        builder =
+            new GrDiffBuilder(
+                {content: content}, {left: [], right: []}, prefs, outputEl);
+        builder.buildSectionElement = function(group) {
+          var section = document.createElement('stub');
+          section.textContent = group.lines.reduce(function(acc, line) {
+            return acc + line.text;
+          }, '');
+          return section;
+        };
+        builder.emitDiff();
+      });
+
+      test('renderSection', function() {
+        var section = outputEl.querySelector('stub:nth-of-type(2)');
+        var prevInnerHTML = section.innerHTML;
+        section.innerHTML = 'wiped';
+        builder.renderSection(section);
+        section = outputEl.querySelector('stub:nth-of-type(2)');
+        assert.equal(section.innerHTML, prevInnerHTML);
+      });
+
+      test('getSectionsByLineRange', function() {
+        var section = outputEl.querySelector('stub:nth-of-type(2)');
+        var sections = builder.getSectionsByLineRange(1, 1, 'left');
+        assert.equal(sections.length, 1);
+        assert.strictEqual(sections[0], section);
+      });
+    });
   });
 </script>
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-group.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-group.js
index 750f7da..7c7c508 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-group.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-group.js
@@ -25,6 +25,8 @@
     }
   }
 
+  GrDiffGroup.prototype.element = null;
+
   GrDiffGroup.Type = {
     BOTH: 'both',
     CONTEXT_CONTROL: 'contextControl',
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-line.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-line.js
index ea00a3d..4acde0c 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-line.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-line.js
@@ -16,13 +16,14 @@
 
   function GrDiffLine(type) {
     this.type = type;
-    this.contextLines = [];
     this.highlights = [];
   }
 
+  GrDiffLine.prototype.afterNumber = 0;
+
   GrDiffLine.prototype.beforeNumber = 0;
 
-  GrDiffLine.prototype.afterNumber = 0;
+  GrDiffLine.prototype.contextGroup = null;
 
   GrDiffLine.prototype.text = '';
 
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
index ea7111a..d813933 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
@@ -215,7 +215,7 @@
       } else {
         var patchNum = this.patchRange.patchNum;
         var side = 'REVISION';
-        if (contentEl.classList.contains(DiffSide.LEFT) ||
+        if (lineEl.classList.contains(DiffSide.LEFT) ||
             contentEl.classList.contains('remove')) {
           if (this.patchRange.basePatchNum === 'PARENT') {
             side = 'PARENT';
@@ -364,6 +364,13 @@
     },
 
     _showContext: function(group, sectionEl) {
+      var groups = this._builder._groups;
+      // TODO(viktard): Polyfill findIndex for IE10.
+      var contextIndex = groups.findIndex(function(group) {
+        return group.element == sectionEl;
+      });
+      groups[contextIndex] = group;
+
       this._builder.emitGroup(group, sectionEl);
       sectionEl.parentNode.removeChild(sectionEl);