Suppress scrolls that begin on fixed-pos layers.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 60272111685995e6722c2cc29f1a464fc224552f..e4e7f8dec1f203ffe5e16cdf51cfb4cf77e1d3a8 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1927,6 +1927,8 @@ void LayerTreeHostImpl::BindToClient(InputHandlerClient* client) {
 }
 
 static LayerImpl* NextScrollLayer(LayerImpl* layer) {
+  if (layer->position_constraint().is_fixed_position())
+    return NULL;
   if (LayerImpl* scroll_parent = layer->scroll_parent())
     return scroll_parent;
   return layer->parent();
@@ -1934,7 +1936,7 @@ static LayerImpl* NextScrollLayer(LayerImpl* layer) {
 
 LayerImpl* LayerTreeHostImpl::FindScrollLayerForViewportPoint(
     gfx::Point viewport_point, InputHandler::ScrollInputType type,
-    bool* scroll_on_main_thread) {
+    bool* scroll_on_main_thread) const {
   DCHECK(scroll_on_main_thread);
 
   gfx::PointF device_viewport_point = gfx::ScalePoint(viewport_point,
@@ -1967,17 +1969,9 @@ LayerImpl* LayerTreeHostImpl::FindScrollLayerForViewportPoint(
       return NULL;
     }
 
-    if (status == ScrollStarted && !potentially_scrolling_layer_impl)
-      potentially_scrolling_layer_impl = scroll_layer_impl;
-  }
-
-  // When hiding top controls is enabled and the controls are hidden or
-  // overlaying the content, force scrolls to be enabled on the root layer to
-  // allow bringing the top controls back into view.
-  if (!potentially_scrolling_layer_impl && top_controls_manager_ &&
-      top_controls_manager_->content_top_offset() !=
-      settings_.top_controls_height) {
-    potentially_scrolling_layer_impl = RootScrollLayer();
+     if ((layer_impl == RootScrollLayer() || status == ScrollStarted) &&
+         !potentially_scrolling_layer_impl)
+       potentially_scrolling_layer_impl = scroll_layer_impl;
   }
 
   return potentially_scrolling_layer_impl;
@@ -2006,10 +2000,6 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     return ScrollOnMainThread;
   }
 
-  // If we want to send a DidOverscroll for this scroll it can't be ignored.
-  if (!potentially_scrolling_layer_impl && settings_.always_overscroll)
-    potentially_scrolling_layer_impl = RootScrollLayer();
-
   if (potentially_scrolling_layer_impl) {
     active_tree_->SetCurrentlyScrollingLayer(
         potentially_scrolling_layer_impl);
@@ -2122,8 +2112,12 @@ bool LayerTreeHostImpl::ScrollBy(gfx::Point viewport_point,
   for (LayerImpl* layer_impl = CurrentlyScrollingLayer();
        layer_impl;
        layer_impl = layer_impl->parent()) {
-    if (!layer_impl->scrollable())
+    if (!layer_impl->scrollable()) {
+      if (layer_impl->position_constraint().is_fixed_position()) {
+        break;
+      }
       continue;
+    }
 
     if (layer_impl == RootScrollLayer()) {
       // Only allow bubble scrolling when the scroll is in the direction to make
@@ -2158,10 +2152,13 @@ bool LayerTreeHostImpl::ScrollBy(gfx::Point viewport_point,
     did_scroll_x |= did_move_layer_x;
     did_scroll_y |= did_move_layer_y;
     if (!did_move_layer_x && !did_move_layer_y) {
-      if (should_bubble_scrolls_ || !did_lock_scrolling_layer_)
+      if (layer_impl->position_constraint().is_fixed_position())
+        break;
+      if (should_bubble_scrolls_ || !did_lock_scrolling_layer_) {
         continue;
-      else
+      } else {
         break;
+      }
     }
 
     if (layer_impl == RootScrollLayer())