[NTP] Tweak app drag and drop.

chrome/browser/resources/ntp/apps.js

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var MAX_APPS_PER_ROW = [];
 MAX_APPS_PER_ROW[LayoutMode.SMALL] = 4;
 MAX_APPS_PER_ROW[LayoutMode.NORMAL] = 6;
 
 // The URL prefix used in the app link 'ping' attributes.
 var PING_APP_LAUNCH_PREFIX = 'record-app-launch';
@@ skipping 349 matching lines @@
 
       return this.dimensions_;
     },
 
     // Gets the item under the mouse event |e|. Returns null if there is no
     // item or if the item is not draggable.
     getItem: function(e) {
       var item = findAncestorByClass(e.target, 'app');
 
       // You can't drag the web store launcher.
-      if (item.classList.contains('web-store-entry'))
+      if (item && item.classList.contains('web-store-entry'))
         return null;
 
       return item;
     },
 
     // Returns true if |coordinates| point to a valid drop location. The
     // coordinates are relative to the drag container and the object should
     // have the 'x' and 'y' properties set.
     canDropOn: function(coordinates) {
       var cols = MAX_APPS_PER_ROW[layoutMode];
@@ skipping 22 matching lines @@
 
       if (current == position || current < 0)
         return;
 
       arrayMove(this.data, current, position);
       this.invalidate_();
       this.layout();
     },
 
     getIndexAt_: function(coordinates) {
-      var x = coordinates.x;
-      var y = coordinates.y;
-
       var w = this.dimensions.width;
       var h = this.dimensions.height;
 
       var availableWidth = this.dragContainer.offsetWidth;
 
-      var row = Math.floor(y / h);
-      var col = Math.floor(x / w);
+      var row = Math.floor(coordinates.y / h);
+      var col = Math.floor(coordinates.x / w);
       var index = Math.floor(availableWidth / w) * row + col;
 
+      var appCount = this.data.length;
+      var cols = MAX_APPS_PER_ROW[layoutMode];
+      var rows = Math.ceil(appCount / cols);
+
+      // Rather than making the free space on the last row invalid, we
+      // map it to the last valid position.
+      if (index >= appCount && index < cols * rows)
+        return appCount-1;
+
       return index;
     },
 
     saveDrag: function() {
       this.invalidate_();
       this.layout();
 
       var appIds = this.data.filter(function(id) {
           return id != 'web-store-entry';
       });
@@ skipping 167 matching lines @@
       a.href = localStrings.getString('web_store_url');
       a.style.backgroundImage = url('chrome://theme/IDR_PRODUCT_LOGO_16');
       a.className = 'item';
       return a;
     }
   };
 })();
 
 // Enable drag and drop reordering of the app launcher.
 var appDragAndDrop = new DragAndDropController(apps);