[DevTools] Reduce relayouts in DataGrid and ViewportControl.

third_party/WebKit/Source/devtools/front_end/ui/ViewportControl.js

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 254 matching lines @@
             node: node,
             offset: offset
         };
     },
 
     /**
      * @param {?Selection} selection
      */
     _updateSelectionModel: function(selection)
     {
-        if (!selection || !selection.rangeCount) {
+        if (!selection || !selection.rangeCount || selection.isCollapsed) {
             this._headSelection = null;
             this._anchorSelection = null;
             return false;
         }
 
         var firstSelected = Number.MAX_VALUE;
         var lastSelected = -1;
 
         var range = selection.getRangeAt(0);
         var hasVisibleSelection = false;
@@ skipping 326 matching lines @@
 
     /**
      * @return {number}
      */
     _visibleHeight: function()
     {
         // Use offsetHeight instead of clientHeight to avoid being affected by horizontal scroll.
         return this.element.offsetHeight;
     }
 }