Respect main thread scrolling reasons for fallback to root layer scrolling.

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 <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 2520 matching lines @@
     scroll_status.thread = InputHandler::SCROLL_IGNORED;
     scroll_status.main_thread_scrolling_reasons =
         MainThreadScrollingReason::kNotScrollable;
     return scroll_status;
   }
 
   scroll_status.thread = InputHandler::SCROLL_ON_IMPL_THREAD;
   return scroll_status;
 }
 
+static bool IsMainThreadScrolling(const InputHandler::ScrollStatus& status,
+                                  const ScrollNode* scroll_node) {
+  if (status.thread == InputHandler::SCROLL_ON_MAIN_THREAD) {
+    if (!!scroll_node->data.main_thread_scrolling_reasons) {
+      DCHECK(MainThreadScrollingReason::MainThreadCanSetScrollReasons(
+          status.main_thread_scrolling_reasons));
+    } else {
+      DCHECK(MainThreadScrollingReason::CompositorCanSetScrollReasons(
+          status.main_thread_scrolling_reasons));
+    }
+    return true;
+  }
+  return false;
+}
+
 LayerImpl* LayerTreeHostImpl::FindScrollLayerForDeviceViewportPoint(
     const gfx::PointF& device_viewport_point,
     InputHandler::ScrollInputType type,
     LayerImpl* layer_impl,
     bool* scroll_on_main_thread,
     uint32_t* main_thread_scrolling_reasons) const {
   DCHECK(scroll_on_main_thread);
   DCHECK(main_thread_scrolling_reasons);
   *main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
 
   // Walk up the hierarchy and look for a scrollable layer.
+  ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   LayerImpl* potentially_scrolling_layer_impl = NULL;
   if (layer_impl) {
-    ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
     ScrollNode* scroll_node = scroll_tree.Node(layer_impl->scroll_tree_index());
     for (; scroll_tree.parent(scroll_node);
          scroll_node = scroll_tree.parent(scroll_node)) {
       // The content layer can also block attempts to scroll outside the main
       // thread.
       ScrollStatus status =
           TryScroll(device_viewport_point, type, scroll_tree, scroll_node);
-      if (status.thread == SCROLL_ON_MAIN_THREAD) {
-        if (!!scroll_node->data.main_thread_scrolling_reasons) {
-          DCHECK(MainThreadScrollingReason::MainThreadCanSetScrollReasons(
-              status.main_thread_scrolling_reasons));
-        } else {
-          DCHECK(MainThreadScrollingReason::CompositorCanSetScrollReasons(
-              status.main_thread_scrolling_reasons));
-        }
-
+      if (IsMainThreadScrolling(status, scroll_node)) {
         *scroll_on_main_thread = true;
         *main_thread_scrolling_reasons = status.main_thread_scrolling_reasons;
         return NULL;
       }
 
       if (status.thread == InputHandler::SCROLL_ON_IMPL_THREAD &&
           !potentially_scrolling_layer_impl) {
         potentially_scrolling_layer_impl =
             active_tree_->LayerById(scroll_node->owner_id);
       }
     }
   }
+
   // Falling back to the root scroll layer ensures generation of root overscroll
-  // notifications while preventing scroll updates from being unintentionally
-  // forwarded to the main thread. The inner viewport layer represents the
-  // viewport during scrolling.
+  // notifications. The inner viewport layer represents the viewport during
+  // scrolling.
   if (!potentially_scrolling_layer_impl)
     potentially_scrolling_layer_impl = InnerViewportScrollLayer();
 
   // The inner viewport layer represents the viewport.
   if (potentially_scrolling_layer_impl == OuterViewportScrollLayer())
     potentially_scrolling_layer_impl = InnerViewportScrollLayer();
 
+  if (potentially_scrolling_layer_impl) {
+    // Ensure that final layer scrolls on impl thread (crbug.com/625100)
+    ScrollNode* scroll_node =
+        scroll_tree.Node(potentially_scrolling_layer_impl->scroll_tree_index());
+    ScrollStatus status =
+        TryScroll(device_viewport_point, type, scroll_tree, scroll_node);
+    if (IsMainThreadScrolling(status, scroll_node)) {
+      *scroll_on_main_thread = true;
+      *main_thread_scrolling_reasons = status.main_thread_scrolling_reasons;
+      return NULL;
+    }
+  }
+
   return potentially_scrolling_layer_impl;
 }
 
 // Similar to LayerImpl::HasAncestor, but walks up the scroll parents.
 static bool HasScrollAncestor(LayerImpl* child, LayerImpl* scroll_ancestor) {
   DCHECK(scroll_ancestor);
   if (!child)
     return false;
   ScrollTree& scroll_tree =
       child->layer_tree_impl()->property_trees()->scroll_tree;
@@ skipping 1413 matching lines @@
   return task_runner_provider_->HasImplThread();
 }
 
 bool LayerTreeHostImpl::CommitToActiveTree() const {
   // In single threaded mode we skip the pending tree and commit directly to the
   // active tree.
   return !task_runner_provider_->HasImplThread();
 }
 
 }  // namespace cc