diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CodeMirrorDemo.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CodeMirrorDemo.java
index 86d2316435..bbe333f921 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CodeMirrorDemo.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CodeMirrorDemo.java
@@ -20,6 +20,7 @@ import com.google.gerrit.client.changes.CommentApi;
 import com.google.gerrit.client.changes.CommentInfo;
 import com.google.gerrit.client.diff.DiffInfo.Region;
 import com.google.gerrit.client.diff.DiffInfo.Span;
+import com.google.gerrit.client.diff.LineMapper.LineOnOtherInfo;
 import com.google.gerrit.client.projects.ConfigInfoCache;
 import com.google.gerrit.client.rpc.CallbackGroup;
 import com.google.gerrit.client.rpc.GerritCallback;
@@ -33,8 +34,6 @@ import com.google.gerrit.reviewdb.client.Project;
 import com.google.gwt.core.client.JavaScriptObject;
 import com.google.gwt.core.client.JsArray;
 import com.google.gwt.core.client.JsArrayString;
-import com.google.gwt.core.client.Scheduler;
-import com.google.gwt.core.client.Scheduler.ScheduledCommand;
 import com.google.gwt.dom.client.Element;
 import com.google.gwt.dom.client.Style.Unit;
 import com.google.gwt.event.logical.shared.ResizeEvent;
@@ -47,6 +46,7 @@ import com.google.gwt.user.client.rpc.AsyncCallback;
 import net.codemirror.lib.CodeMirror;
 import net.codemirror.lib.CodeMirror.LineClassWhere;
 import net.codemirror.lib.Configuration;
+import net.codemirror.lib.KeyMap;
 import net.codemirror.lib.LineCharacter;
 import net.codemirror.lib.LineWidget;
 import net.codemirror.lib.ModeInjector;
@@ -60,6 +60,11 @@ public class CodeMirrorDemo extends Screen {
   private static final int HEADER_FOOTER = 60 + 15 * 2 + 38;
   private static final JsArrayString EMPTY =
       JavaScriptObject.createArray().cast();
+  /**
+   * TODO: Handle ordering of line widgets. CodeMirror seems to have a bug when
+   * multiple line widgets are added to a line: it puts the first widget above
+   * the line when the line is clicked upon.
+   */
   private static final Configuration COMMENT_BOX_CONFIG =
       Configuration.create().set("coverGutter", true);
 
@@ -78,6 +83,8 @@ public class CodeMirrorDemo extends Screen {
   private LineMapper mapper;
   private CommentLinkProcessor commentLinkProcessor;
   private Map<String, PublishedBox> publishedMap;
+  private Map<Integer, CommentBox> lineBoxMapA;
+  private Map<Integer, CommentBox> lineBoxMapB;
 
   public CodeMirrorDemo(
       PatchSet.Id base,
@@ -160,14 +167,9 @@ public class CodeMirrorDemo extends Screen {
       cmB.refresh();
     }
     Window.enableScrolling(false);
-    Scheduler.get().scheduleDeferred(new ScheduledCommand() {
-      @Override
-      public void execute() {
-        for (CommentBox box : initialBoxes) {
-          box.resizePaddingWidget();
-        }
-      }
-    });
+    for (CommentBox box : initialBoxes) {
+      box.resizePaddingWidget();
+    }
   }
 
   @Override
@@ -187,9 +189,6 @@ public class CodeMirrorDemo extends Screen {
       cmB = null;
     }
     Window.enableScrolling(true);
-    mapper = null;
-    initialBoxes = null;
-    publishedMap = null;
   }
 
   private void display(DiffInfo diffInfo) {
@@ -198,11 +197,16 @@ public class CodeMirrorDemo extends Screen {
     render(diffInfo);
     initialBoxes = new ArrayList<CommentBox>();
     publishedMap = new HashMap<String, PublishedBox>(published.length());
+    lineBoxMapA = new HashMap<Integer, CommentBox>();
+    lineBoxMapB = new HashMap<Integer, CommentBox>();
     renderPublished();
     renderDrafts();
     published = null;
     drafts = null;
-
+    cmA.on("cursorActivity", updateActiveLine(cmA));
+    cmB.on("cursorActivity", updateActiveLine(cmB));
+    cmA.addKeyMap(KeyMap.create().on("'c'", insertNewDraft(cmA)));
+    cmB.addKeyMap(KeyMap.create().on("'c'", insertNewDraft(cmB)));
     // TODO: Probably need horizontal resize
     resizeHandler = Window.addResizeHandler(new ResizeHandler() {
       @Override
@@ -271,16 +275,37 @@ public class CodeMirrorDemo extends Screen {
     }
   }
 
-  DraftBox addReplyBox(CommentInfo replyTo, String initMessage, boolean doSave) {
+  private DraftBox addNewDraft(CodeMirror cm, int line) {
+    Side side = cm == cmA ? Side.PARENT : Side.REVISION;
     CommentInfo info = CommentInfo.create(
         path,
-        replyTo.side(),
-        replyTo.line(),
+        side,
+        line + 1,
+        null,
+        null);
+    return addDraftBox(info, false);
+  }
+
+  DraftBox addReply(CommentInfo replyTo, String initMessage, boolean doSave) {
+    Side side = replyTo.side();
+    int line = replyTo.line();
+    CommentInfo info = CommentInfo.create(
+        path,
+        side,
+        line,
         replyTo.id(),
         initMessage);
+    return addDraftBox(info, doSave);
+  }
+
+  private DraftBox addDraftBox(CommentInfo info, boolean doSave) {
     DraftBox box = new DraftBox(this, revision, info, commentLinkProcessor,
-        true, !doSave);
+        true, doSave);
     addCommentBox(info, box);
+    if (!doSave) {
+      box.setEdit(true);
+    }
+    getLineBoxMapFromSide(info.side()).put(info.line() - 1, box);
     return box;
   }
 
@@ -292,7 +317,7 @@ public class CodeMirrorDemo extends Screen {
     int line = info.line() - 1; // CommentInfo is 1-based, but CM is 0-based
     LineWidget boxWidget =
         cm.addLineWidget(line, box.getElement(), COMMENT_BOX_CONFIG);
-    int lineToPad = mapper.lineOnOther(mySide, line);
+    int lineToPad = mapper.lineOnOther(mySide, line).getLine();
     // Estimated height at 21px, fixed by deferring after display
     LineWidgetElementPair padding = addPaddingWidget(
         other, diffTable.style.padding(), lineToPad, 21, Unit.PX);
@@ -301,14 +326,20 @@ public class CodeMirrorDemo extends Screen {
     return box;
   }
 
+  void removeCommentBox(Side side, int line) {
+    getLineBoxMapFromSide(side).remove(line);
+  }
+
   private void renderPublished() {
     for (int i = 0; published != null && i < published.length(); i++) {
       CommentInfo info = published.get(i);
       final PublishedBox box =
           new PublishedBox(this, revision, info, commentLinkProcessor);
+      box.setOpen(false);
       addCommentBox(info, box);
       initialBoxes.add(box);
       publishedMap.put(info.id(), box);
+      getLineBoxMapFromSide(info.side()).put(info.line() - 1, box);
     }
   }
 
@@ -317,13 +348,15 @@ public class CodeMirrorDemo extends Screen {
       CommentInfo info = drafts.get(i);
       final DraftBox box =
           new DraftBox(this, revision, info, commentLinkProcessor, false, false);
+      box.setOpen(false);
+      box.setEdit(false);
       addCommentBox(info, box);
       initialBoxes.add(box);
       PublishedBox replyToBox = publishedMap.get(info.in_reply_to());
       if (replyToBox != null) {
         replyToBox.registerReplyBox(box);
-        box.registerReplyToBox(replyToBox);
       }
+      getLineBoxMapFromSide(info.side()).put(info.line() - 1, box);
     }
   }
 
@@ -331,6 +364,10 @@ public class CodeMirrorDemo extends Screen {
     return me == cmA ? cmB : cmA;
   }
 
+  private Map<Integer, CommentBox> getLineBoxMapFromSide(Side side) {
+    return side == Side.PARENT ? lineBoxMapA : lineBoxMapB;
+  }
+
   private void markEdit(CodeMirror cm, JsArrayString lines,
       JsArray<Span> edits, int startLine) {
     if (edits == null) {
@@ -396,6 +433,58 @@ public class CodeMirrorDemo extends Screen {
     };
   }
 
+  private Runnable updateActiveLine(final CodeMirror cm) {
+    final CodeMirror other = otherCM(cm);
+    return new Runnable() {
+      public void run() {
+        if (cm.hasActiveLine()) {
+          cm.removeLineClass(cm.getActiveLine(),
+              LineClassWhere.WRAP, diffTable.style.activeLine());
+          cm.removeLineClass(cm.getActiveLine(),
+              LineClassWhere.BACKGROUND, diffTable.style.activeLineBg());
+        }
+        if (other.hasActiveLine()) {
+          other.removeLineClass(other.getActiveLine(),
+              LineClassWhere.WRAP, diffTable.style.activeLine());
+          other.removeLineClass(other.getActiveLine(),
+              LineClassWhere.BACKGROUND, diffTable.style.activeLineBg());
+        }
+        int line = cm.getCursor("head").getLine();
+        LineOnOtherInfo info =
+            mapper.lineOnOther(cm == cmA ? Side.PARENT : Side.REVISION, line);
+        int oLine = info.getLine();
+        cm.setActiveLine(line);
+        cm.addLineClass(line, LineClassWhere.WRAP, diffTable.style.activeLine());
+        cm.addLineClass(line, LineClassWhere.BACKGROUND, diffTable.style.activeLineBg());
+        if (info.isAligned()) {
+          other.setActiveLine(oLine);
+          other.addLineClass(oLine, LineClassWhere.WRAP,
+              diffTable.style.activeLine());
+          other.addLineClass(oLine, LineClassWhere.BACKGROUND,
+              diffTable.style.activeLineBg());
+        }
+      }
+    };
+  }
+
+  private Runnable insertNewDraft(final CodeMirror cm) {
+    return new Runnable() {
+      public void run() {
+        Map<Integer, CommentBox> lineBoxMap = cm == cmA ?
+            lineBoxMapA : lineBoxMapB;
+        int line = cm.getActiveLine();
+        CommentBox box = lineBoxMap.get(line);
+        if (box == null) {
+          lineBoxMap.put(line, addNewDraft(cm, line));
+        } else if (box.isDraft()) {
+          ((DraftBox) lineBoxMap.get(line)).setEdit(true);
+        } else {
+          ((PublishedBox) box).onReply(null);
+        }
+      }
+    };
+  }
+
   private static String getContentType(DiffInfo.FileMeta meta) {
     return meta != null && meta.content_type() != null
         ? ModeInjector.getContentType(meta.content_type())
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentBox.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentBox.java
index aa5c0c1712..3ba3a0f937 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentBox.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentBox.java
@@ -17,6 +17,8 @@ package com.google.gerrit.client.diff;
 import com.google.gerrit.client.changes.CommentInfo;
 import com.google.gerrit.client.ui.CommentLinkProcessor;
 import com.google.gerrit.reviewdb.client.PatchSet;
+import com.google.gwt.core.client.Scheduler;
+import com.google.gwt.core.client.Scheduler.ScheduledCommand;
 import com.google.gwt.dom.client.Element;
 import com.google.gwt.dom.client.Style.Unit;
 import com.google.gwt.event.dom.client.ClickEvent;
@@ -50,6 +52,7 @@ abstract class CommentBox extends Composite {
   private LineWidget paddingWidget;
   private Element paddingWidgetEle;
   private CodeMirrorDemo diffView;
+  private boolean draft;
 
   @UiField(provided=true)
   CommentBoxHeader header;
@@ -69,10 +72,10 @@ abstract class CommentBox extends Composite {
     commentLinkProcessor = linkProcessor;
     original = info;
     patchSetId = id;
+    draft = isDraft;
     header = new CommentBoxHeader(info.author(), info.updated(), isDraft);
     initWidget(binder.createAndBindUi(this));
     setMessageText(info.message());
-    setOpen(false);
   }
 
   @Override
@@ -83,7 +86,6 @@ abstract class CommentBox extends Composite {
       @Override
       public void onClick(ClickEvent event) {
         setOpen(!isOpen());
-        resizePaddingWidget();
       }
     }, ClickEvent.getType());
     res.style().ensureInjected();
@@ -109,7 +111,14 @@ abstract class CommentBox extends Composite {
   }
 
   void resizePaddingWidget() {
-    paddingWidgetEle.getStyle().setHeight(getOffsetHeight(), Unit.PX);
+    Scheduler.get().scheduleDeferred(new ScheduledCommand() {
+      @Override
+      public void execute() {
+        paddingWidgetEle.getStyle().setHeight(getOffsetHeight(), Unit.PX);
+        paddingWidget.changed();
+        selfWidget.changed();
+      }
+    });
   }
 
   protected void setMessageText(String message) {
@@ -136,6 +145,7 @@ abstract class CommentBox extends Composite {
       removeStyleName(res.style().open());
       addStyleName(res.style().close());
     }
+    resizePaddingWidget();
   }
 
   private boolean isOpen() {
@@ -165,4 +175,8 @@ abstract class CommentBox extends Composite {
   protected void updateOriginal(CommentInfo newInfo) {
     original = newInfo;
   }
+
+  boolean isDraft() {
+    return draft;
+  }
 }
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentBoxHeader.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentBoxHeader.java
index 9b703330c2..5ffd925ee8 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentBoxHeader.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentBoxHeader.java
@@ -36,7 +36,7 @@ class CommentBoxHeader extends Composite {
   interface Binder extends UiBinder<HTMLPanel, CommentBoxHeader> {}
   private static Binder uiBinder = GWT.create(Binder.class);
 
-  private boolean isDraft;
+  private boolean draft;
 
   @UiField(provided=true)
   AvatarImage avatar;
@@ -52,9 +52,9 @@ class CommentBoxHeader extends Composite {
 
   CommentBoxHeader(AccountInfo author, Timestamp when, boolean isDraft) {
     // TODO: Set avatar's display to none if we get a 404.
-    avatar = new AvatarImage(author, 26);
+    avatar = author == null ? new AvatarImage() : new AvatarImage(author, 26);
     initWidget(uiBinder.createAndBindUi(this));
-    this.isDraft = isDraft;
+    this.draft = isDraft;
     if (when != null) {
       setDate(when);
     }
@@ -66,7 +66,7 @@ class CommentBoxHeader extends Composite {
   }
 
   void setDate(Timestamp when) {
-    if (isDraft) {
+    if (draft) {
       date.setInnerText(PatchUtil.M.draftSaved(when));
     } else {
       date.setInnerText(FormatUtil.shortFormatDayTime(when));
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
index a177498312..0e22876083 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
@@ -35,6 +35,8 @@ class DiffTable extends Composite {
     String diff();
     String intraline();
     String padding();
+    String activeLine();
+    String activeLineBg();
   }
 
   @UiField
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml
index 0e303bcceb..7608267069 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml
@@ -57,6 +57,12 @@ limitations under the License.
     .a .CodeMirror-vscrollbar {
       display: none !important;
     }
+    .activeLine .CodeMirror-linenumber {
+      background-color: #D8EDF9;
+    }
+    .activeLineBg {
+      background-color: #D8EDF9;
+    }
   </ui:style>
   <g:HTMLPanel styleName='{style.difftable}'>
     <table class='{style.table}'>
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DraftBox.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DraftBox.java
index d452fe2cae..c4906de2cb 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DraftBox.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DraftBox.java
@@ -81,8 +81,6 @@ class DraftBox extends CommentBox {
 
     this.isNew = isNew;
     editArea.setText(contentPanelMessage.getText());
-    setEdit(isNew && !saveOnInit);
-    setOpen(isNew && !saveOnInit);
     if (saveOnInit) {
       onSave(null);
     }
@@ -120,6 +118,7 @@ class DraftBox extends CommentBox {
 
   void setEdit(boolean edit) {
     if (edit) {
+      setOpen(true);
       removeStyleName(draftStyle.view());
       addStyleName(draftStyle.edit());
       editArea.setText(contentPanelMessage.getText());
@@ -139,9 +138,11 @@ class DraftBox extends CommentBox {
     if (replyToBox != null) {
       replyToBox.unregisterReplyBox();
     }
-    getPaddingWidget().clear();
+    CommentInfo info = getOriginal();
+    getDiffView().removeCommentBox(info.side(), info.line() - 1);
     removeFromParent();
     getSelfWidget().clear();
+    getPaddingWidget().clear();
   }
 
   @UiHandler("edit")
@@ -162,10 +163,9 @@ class DraftBox extends CommentBox {
       @Override
       public void onSuccess(CommentInfo result) {
         updateOriginal(result);
-        setEdit(false);
         setMessageText(message);
         setDate(result.updated());
-        resizePaddingWidget();
+        setEdit(false);
         if (isNew) {
           removeStyleName(draftStyle.newDraft());
           isNew = false;
@@ -182,7 +182,6 @@ class DraftBox extends CommentBox {
   @UiHandler("cancel")
   void onCancel(ClickEvent e) {
     setEdit(false);
-    resizePaddingWidget();
   }
 
   @UiHandler("discard")
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/LineMapper.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/LineMapper.java
index b86f59fe34..aed0813c77 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/LineMapper.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/LineMapper.java
@@ -49,7 +49,7 @@ class LineMapper {
     int origLineB = lineB;
     lineB += numLines;
     int bAheadOfA = lineB - lineA;
-    lineMapAtoB.add(new LineGap(lineA, lineA, bAheadOfA));
+    lineMapAtoB.add(new LineGap(lineA, -1, bAheadOfA));
     lineMapBtoA.add(new LineGap(origLineB, lineB - 1, -bAheadOfA));
   }
 
@@ -58,7 +58,7 @@ class LineMapper {
     lineA += numLines;
     int aAheadOfB = lineA - lineB;
     lineMapAtoB.add(new LineGap(origLineA, lineA - 1, -aAheadOfB));
-    lineMapBtoA.add(new LineGap(lineB, lineB, aAheadOfB));
+    lineMapBtoA.add(new LineGap(lineB, -1, aAheadOfB));
   }
 
   /**
@@ -69,8 +69,9 @@ class LineMapper {
    *
    * A LineGap records gap information from the start of an actual gap up to
    * the start of the next gap. In the following example,
-   * lineMapAtoB will have LineGap: {start: 1, end: 1, delta: 3}
-   * (end doesn't really matter here, as the binary search only looks at start)
+   * lineMapAtoB will have LineGap: {start: 1, end: -1, delta: 3}
+   * (end set to -1 to represent a dummy gap of length zero. The binary search
+   * only looks at start so setting it to -1 has no effect here.)
    * lineMapBtoA will have LineGap: {start: 1, end: 3, delta: -3}
    * These LineGaps control lines between 1 and 5.
    *
@@ -97,24 +98,61 @@ class LineMapper {
    *   -   |   6
    *      ...
    */
-  int lineOnOther(Side mySide, int line) {
+  LineOnOtherInfo lineOnOther(Side mySide, int line) {
     List<LineGap> lineGaps = mySide == Side.PARENT ? lineMapAtoB : lineMapBtoA;
     // Create a dummy LineGap for the search.
     int ret = Collections.binarySearch(lineGaps, new LineGap(line));
     if (ret == -1) {
-      return line;
+      return new LineOnOtherInfo(line, true);
     } else {
       LineGap lookup = lineGaps.get(0 <= ret ? ret : -ret - 2);
+      int start = lookup.start;
       int end = lookup.end;
       int delta = lookup.delta;
-      if (lookup.start <= line && line <= end) { // Line falls within gap
-        return end + delta;
+      if (start <= line && line <= end && end != -1) { // Line falls within gap
+        return new LineOnOtherInfo(end + delta, false);
       } else { // Line after gap
-        return line + delta;
+        return new LineOnOtherInfo(line + delta, true);
       }
     }
   }
 
+  /**
+   * @field line The line number on the other side.
+   * @field aligned Whether the two lines are at the same height when displayed.
+   */
+  static class LineOnOtherInfo {
+    private int line;
+    private boolean aligned;
+
+    LineOnOtherInfo(int line, boolean aligned) {
+      this.line = line;
+      this.aligned = aligned;
+    }
+
+    int getLine() {
+      return line;
+    }
+
+    boolean isAligned() {
+      return aligned;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+      if (obj instanceof LineOnOtherInfo) {
+        LineOnOtherInfo other = (LineOnOtherInfo) obj;
+        return aligned == other.aligned && line == other.line;
+      }
+      return false;
+    }
+
+    @Override
+    public String toString() {
+      return line + " " + aligned;
+    }
+  }
+
   /**
    * Helper class to record line gap info and assist in calculation of line
    * number on the other side.
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PublishedBox.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PublishedBox.java
index 50da896a7c..337b52840d 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PublishedBox.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PublishedBox.java
@@ -53,7 +53,7 @@ class PublishedBox extends CommentBox {
   @UiHandler("reply")
   void onReply(ClickEvent e) {
     if (replyBox == null) {
-      DraftBox box = getDiffView().addReplyBox(getOriginal(), "", true);
+      DraftBox box = getDiffView().addReply(getOriginal(), "", false);
       registerReplyBox(box);
     } else {
       openReplyBox();
@@ -63,7 +63,7 @@ class PublishedBox extends CommentBox {
   @UiHandler("replyDone")
   void onReplyDone(ClickEvent e) {
     if (replyBox == null) {
-      DraftBox box = getDiffView().addReplyBox(getOriginal(), "Done", false);
+      DraftBox box = getDiffView().addReply(getOriginal(), "Done", true);
       registerReplyBox(box);
     } else {
       openReplyBox();
diff --git a/gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java b/gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java
index dbf1a2bdab..69d1671d91 100644
--- a/gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java
+++ b/gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java
@@ -66,6 +66,16 @@ public class CodeMirror extends JavaScriptObject {
     this.addLineClass(line, where, lineClass);
   }-*/;
 
+  public final void removeLineClass(int line, LineClassWhere where,
+      String className) {
+    removeLineClassNative(line, where.name().toLowerCase(), className);
+  }
+
+  private final native void removeLineClassNative(int line, String where,
+      String lineClass) /*-{
+    this.removeLineClass(line, where, lineClass);
+  }-*/;
+
   public final native void addWidget(LineCharacter pos, Element node,
       boolean scrollIntoView) /*-{
     this.addWidget(pos, node, scrollIntoView);
@@ -102,6 +112,24 @@ public class CodeMirror extends JavaScriptObject {
     });
   }-*/;
 
+  public final native LineCharacter getCursor(String start) /*-{
+    return this.getCursor(start);
+  }-*/;
+
+  public final native boolean hasActiveLine() /*-{
+    return this.state.hasOwnProperty('activeLine');
+  }-*/;
+
+  public final native int getActiveLine() /*-{
+    return this.state.activeLine;
+  }-*/;
+
+  public final native void setActiveLine(int line) /*-{
+    this.state.activeLine = line;
+  }-*/;
+
+  public final native void addKeyMap(KeyMap map) /*-{ this.addKeyMap(map); }-*/;
+
   protected CodeMirror() {
   }
 }
diff --git a/gerrit-gwtui/src/main/java/net/codemirror/lib/KeyMap.java b/gerrit-gwtui/src/main/java/net/codemirror/lib/KeyMap.java
new file mode 100644
index 0000000000..ac2ca5cd3e
--- /dev/null
+++ b/gerrit-gwtui/src/main/java/net/codemirror/lib/KeyMap.java
@@ -0,0 +1,32 @@
+// Copyright (C) 2013 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package net.codemirror.lib;
+
+import com.google.gwt.core.client.JavaScriptObject;
+
+/** Object that associates a key or key combination with a handler. */
+public class KeyMap extends JavaScriptObject {
+  public static KeyMap create() {
+    return createObject().cast();
+  }
+
+  public final native KeyMap on(String key, Runnable thunk) /*-{
+    this[key] = function() { $entry(thunk.@java.lang.Runnable::run()()); };
+    return this;
+  }-*/;
+
+  protected KeyMap() {
+  }
+}
diff --git a/gerrit-gwtui/src/test/java/com/google/gerrit/client/diff/LineMapperTest.java b/gerrit-gwtui/src/test/java/com/google/gerrit/client/diff/LineMapperTest.java
index 74405e9b30..96ba2b7dbb 100644
--- a/gerrit-gwtui/src/test/java/com/google/gerrit/client/diff/LineMapperTest.java
+++ b/gerrit-gwtui/src/test/java/com/google/gerrit/client/diff/LineMapperTest.java
@@ -16,6 +16,7 @@ package com.google.gerrit.client.diff;
 
 import static org.junit.Assert.assertEquals;
 
+import com.google.gerrit.client.diff.LineMapper.LineOnOtherInfo;
 import com.google.gerrit.common.changes.Side;
 
 import org.junit.Test;
@@ -51,41 +52,55 @@ public class LineMapperTest {
   public void testFindInCommon() {
     LineMapper mapper = new LineMapper();
     mapper.appendCommon(10);
-    assertEquals(9, mapper.lineOnOther(Side.PARENT, 9));
+    assertEquals(new LineOnOtherInfo(9, true),
+        mapper.lineOnOther(Side.PARENT, 9));
+    assertEquals(new LineOnOtherInfo(9, true),
+        mapper.lineOnOther(Side.REVISION, 9));
   }
 
   @Test
   public void testFindAfterCommon() {
     LineMapper mapper = new LineMapper();
     mapper.appendCommon(10);
-    assertEquals(10, mapper.lineOnOther(Side.PARENT, 10));
+    assertEquals(new LineOnOtherInfo(10, true),
+        mapper.lineOnOther(Side.PARENT, 10));
+    assertEquals(new LineOnOtherInfo(10, true),
+        mapper.lineOnOther(Side.REVISION, 10));
   }
 
   @Test
   public void testFindInInsertGap() {
     LineMapper mapper = new LineMapper();
     mapper.appendInsert(10);
-    assertEquals(-1, mapper.lineOnOther(Side.REVISION, 9));
+    assertEquals(new LineOnOtherInfo(-1, false),
+        mapper.lineOnOther(Side.REVISION, 9));
   }
 
   @Test
   public void testFindAfterInsertGap() {
     LineMapper mapper = new LineMapper();
     mapper.appendInsert(10);
-    assertEquals(0, mapper.lineOnOther(Side.REVISION, 10));
+    assertEquals(new LineOnOtherInfo(0, true),
+        mapper.lineOnOther(Side.REVISION, 10));
+    assertEquals(new LineOnOtherInfo(10, true),
+        mapper.lineOnOther(Side.PARENT, 0));
   }
 
   @Test
   public void testFindInDeleteGap() {
     LineMapper mapper = new LineMapper();
     mapper.appendDelete(10);
-    assertEquals(-1, mapper.lineOnOther(Side.PARENT, 9));
+    assertEquals(new LineOnOtherInfo(-1, false),
+        mapper.lineOnOther(Side.PARENT, 9));
   }
 
   @Test
   public void testFindAfterDeleteGap() {
     LineMapper mapper = new LineMapper();
     mapper.appendDelete(10);
-    assertEquals(0, mapper.lineOnOther(Side.PARENT, 10));
+    assertEquals(new LineOnOtherInfo(0, true),
+        mapper.lineOnOther(Side.PARENT, 10));
+    assertEquals(new LineOnOtherInfo(10, true),
+        mapper.lineOnOther(Side.REVISION, 0));
   }
 }