Properly handle viewport-flings after a bubbling scroll

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 2663 matching lines @@
     did_scroll_x |= did_move_layer_x;
     did_scroll_y |= did_move_layer_y;
     if (!did_move_layer_x && !did_move_layer_y) {
       if (allow_bubbling_for_current_layer || !did_lock_scrolling_layer_)
         continue;
       else
         break;
     }
 
     did_lock_scrolling_layer_ = true;
-    if (!allow_bubbling_for_current_layer) {
+
+    // When scrolls are allowed to bubble, it's important that the original
+    // scrolling layer be preserved. This ensures that, after a scroll bubbles,
+    // the user can reverse scroll directions and immediately resume scrolling
+    // the original layer that scrolled.
+    if (!should_bubble_scrolls_)
       active_tree_->SetCurrentlyScrollingLayer(layer_impl);
+
+    if (!allow_bubbling_for_current_layer)
       break;
-    }
 
     if (allow_unrestricted_bubbling_for_current_layer) {
       pending_delta -= applied_delta;
     } else {
       // If the applied delta is within 45 degrees of the input delta, bail out
       // to make it easier to scroll just one layer in one direction without
       // affecting any of its parents.
       float angle_threshold = 45;
       if (MathUtil::SmallestAngleBetweenVectors(applied_delta, pending_delta) <
           angle_threshold) {
@@ skipping 779 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc