Always fall back to root scroll layer for otherwise ignored scrolls

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 f9076fb9be87def8080895959ff2be042118032b..5bc1fc618cad8b8d5ba01f504b278e7f6665c04b 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2009,15 +2009,6 @@ LayerImpl* LayerTreeHostImpl::FindScrollLayerForDeviceViewportPoint(
       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();
-  }
-
   return potentially_scrolling_layer_impl;
 }
 
@@ -2049,8 +2040,7 @@ 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)
+  if (!potentially_scrolling_layer_impl)
     potentially_scrolling_layer_impl = RootScrollLayer();
 
   if (potentially_scrolling_layer_impl) {
@@ -2158,8 +2148,19 @@ bool LayerTreeHostImpl::ScrollBy(gfx::Point viewport_point,
   gfx::Vector2dF unused_root_delta;
   bool did_scroll_x = false;
   bool did_scroll_y = false;
-  bool consume_by_top_controls = top_controls_manager_ &&
-      (CurrentlyScrollingLayer() == RootScrollLayer() || scroll_delta.y() < 0);
+  bool consume_by_top_controls = false;
+  if (top_controls_manager_) {
+    gfx::PointF screen_space_point =
+        gfx::ScalePoint(viewport_point, device_scale_factor_);

[aelias_OOO_until_Jul13, 2013/12/26 20:38:00]

This would be incorrect when the transformation is more complex than this (for
example, in some WebView and devtools scenarios).  We should be reusing the
logic in ScrollLayerWithViewportSpaceDelta for this.  So probably the top
controls consumption logic needs to move to that method as well.

[jdduke (slow), 2014/01/09 20:37:29]

What if we simply use the |max_scroll_offset()|?  I tested this with the latest
patch set, and it seems to work really well in practice. 

+    bool root_layer_gesture_scroll_active_and_possible =
+        CurrentlyScrollingLayer() == RootScrollLayer() &&
+        RootScrollLayer()->TryScroll(screen_space_point, Gesture)
+            != ScrollIgnored;
+    // Showing the top controls should always be possible, while hiding them
+    // should be possible only if the root layer can be scrolled.
+    consume_by_top_controls =
+      (scroll_delta.y() < 0 || root_layer_gesture_scroll_active_and_possible);
+  }
 
   for (LayerImpl* layer_impl = CurrentlyScrollingLayer();
        layer_impl;
@@ -2170,7 +2171,7 @@ bool LayerTreeHostImpl::ScrollBy(gfx::Point viewport_point,
     if (layer_impl == RootScrollLayer()) {
       // Only allow bubble scrolling when the scroll is in the direction to make
       // the top controls visible.
-      if (consume_by_top_controls && layer_impl == RootScrollLayer()) {
+      if (consume_by_top_controls) {
         pending_delta = top_controls_manager_->ScrollBy(pending_delta);
         UpdateMaxScrollOffset();
       }