SideBySide2: Batch updates to CodeMirror3 during onShowView

Colby recently observed opening files is sluggish.  Digging into
profile data shows CM3 doing multiple relayouts if its internal
DOM during onShowView(), once for each method being invoked. Batch
these together into an operation() call, allowing CM3 to buffer
the updates and update its DOM only once.

Change-Id: Iefbc112a37016c4031515b4422a02368e5af16c1

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySide2.java

@@ -231,17 +231,29 @@ public class SideBySide2 extends Screen {
   @Override
   public void onShowView() {
     super.onShowView();
-    resizeCodeMirror();
     Window.enableScrolling(false);
 
-    cmA.setOption("viewportMargin", 10);
-    cmB.setOption("viewportMargin", 10);
+    final int height = getCodeMirrorHeight();
+    onShowView(cmA, height);
+    onShowView(cmB, height);
+    diffTable.sidePanel.adjustGutters(cmB);
     cmB.setCursor(LineCharacter.create(0));
     cmB.focus();
 
     prefetchNextFile();
   }
 
+  private void onShowView(final CodeMirror cm, final int height) {
+    cm.operation(new Runnable() {
+      @Override
+      public void run() {
+        cm.setHeight(height);
+        cm.setOption("viewportMargin", 10);
+        cm.refresh();
+      }
+    });
+  }
+
   @Override
   protected void onUnload() {
     super.onUnload();
@@ -1239,16 +1251,18 @@ public class SideBySide2 extends Screen {
   }
 
   void resizeCodeMirror() {
+    int height = getCodeMirrorHeight();
+    cmA.setHeight(height);
+    cmB.setHeight(height);
+    diffTable.sidePanel.adjustGutters(cmB);
+  }
+
+  private int getCodeMirrorHeight() {
     int rest = Gerrit.getHeaderFooterHeight()
         + header.getOffsetHeight()
         + diffTable.getHeaderHeight()
         + 5; // Estimate
-    int h = Window.getClientHeight() - rest;
-    cmA.setHeight(h);
-    cmB.setHeight(h);
-    cmA.refresh();
-    cmB.refresh();
-    diffTable.sidePanel.adjustGutters(cmB);
+    return Window.getClientHeight() - rest;
   }
 
   static void setHeightInPx(Element ele, double height) {

gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java

@@ -166,6 +166,12 @@ public class CodeMirror extends JavaScriptObject {
     this.state.oldViewportSize = lines;
   }-*/;
 
+  public final native void operation(Runnable thunk) /*-{
+    this.operation(function() {
+      thunk.@java.lang.Runnable::run()();
+    });
+  }-*/;
+
   public final native void on(String event, Runnable thunk) /*-{
     this.on(event, $entry(function() {
       thunk.@java.lang.Runnable::run()();