[MD Bookmarks] Allow dragging onto empty bookmark lists.

chrome/test/data/webui/md_bookmarks/dnd_manager_test.js

 // Copyright 2017 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.
 
 suite('drag and drop', function() {
   var app;
   var list;
   var sidebar;
   var store;
   var dndManager;
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     // from.
     dndManager.dragInfo_.handleChromeDragEnter(createDragData(['2']));
     assertEquals(
         DropPosition.NONE,
         dndManager.calculateValidDropPositions_(getListItem('13')));
 
     // Drags onto folders should work as per usual.
     assertEquals(
         DropPosition.ON | DropPosition.ABOVE | DropPosition.BELOW,
         dndManager.calculateValidDropPositions_(getFolderNode('112')));
+
+    // Drags onto an empty search list do nothing.
+    store.data.search.results = [];
+    store.notifyObservers();
+    assertEquals(
+        DropPosition.NONE, dndManager.calculateValidDropPositions_(list));
   });
 
   test('calculateDropInfo_', function() {
     function assertDropInfo(parentId, index, element, position) {
       assertDeepEquals(
           {parentId: parentId, index: index},
           dndManager.calculateDropInfo_(
               {element: element, position: position}));
     }
 
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     dispatchDragEvent('dragover', dragTarget);
     assertEquals(700, autoExpander.lastTimestamp_);
 
     autoExpander.testTimestamp_ += autoExpander.EXPAND_FOLDER_DELAY;
     dispatchDragEvent('dragover', dragTarget);
     assertDeepEquals(
         bookmarks.actions.changeFolderOpen('11', true), store.lastAction);
     assertEquals(0, autoExpander.lastTimestamp_);
     assertEquals(null, autoExpander.lastElement_);
   });
+
+  test('drag onto empty list', function() {
+    store.data.selectedFolder = '14';
+    store.notifyObservers();
+
+    var dragElement = getFolderNode('15');
+    var dragTarget = list;
+
+    // Dragging onto an empty list.
+    dispatchDragEvent('dragstart', dragElement);
+    dndManager.dragInfo_.handleChromeDragEnter(createDragData(draggedIds));
+
+    dispatchDragEvent('dragover', dragTarget);
+    assertEquals(
+        DropPosition.ON, dndManager.calculateValidDropPositions_(dragTarget));
+    assertDragStyle(dragTarget, DRAG_STYLE.ON);
+
+    dispatchDragEvent('dragend', dragTarget);
+
+    // Dragging onto a non-empty list.
+    store.data.selectedFolder = '11';
+    store.notifyObservers();
+
+    dispatchDragEvent('dragstart', dragElement);
+    dndManager.dragInfo_.handleChromeDragEnter(createDragData(draggedIds));
+
+    dispatchDragEvent('dragover', dragTarget);
+    assertEquals(
+        DropPosition.NONE, dndManager.calculateValidDropPositions_(dragTarget));
+    assertDragStyle(dragTarget, DRAG_STYLE.NONE);
+  });
 });