Hide/show topbar when scrolling page up or down.

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 10b06854b8fb1f5367eb3c08021d2442493aaf66..fd72979298a930b965ecfe7370e6d6dfadc5c975 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2570,7 +2570,7 @@ bool LayerTreeHostImpl::ShouldTopControlsConsumeScroll(
     return false;
 
   // Always consume if it's in the direction to show the top controls.
-  if (scroll_delta.y() < 0)
+  if (scroll_delta.y() != 0)

[aelias_OOO_until_Jul13, 2014/12/02 19:55:02]

This fix isn't correct as it will hide the top controls even if the page is
nonscrollable (i.e. this change defeats the purpose of the if statement on line
2580).

It looks like the underlying problem you're fixing is that top controls hiding
doesn't work when scrolls bubble up from a scrollable sublayer.  That wikipedia
table is a scrollable sublayer (that happens to not have any room to scroll). 
Maybe a more correct fix would be to remove the CurrentlyScrollingLayer() != if
statement right below, as it seems that the problem is that we're checking that
too early and it's redundant with a later-binding check.

[elisabets, 2014/12/05 13:30:26]

Ah, so the CurrentlyScrollingLayer is the <scrollable> table (which is not the
same as either viewport). 
It makes a lot more sense to remove the following if-statement :)

On 2014/12/02 19:55:02, aelias wrote:

     return true;
 
   if (CurrentlyScrollingLayer() != InnerViewportScrollLayer() &&