Touchpad scroll latching enabled for Mac behind flag.

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 e2dbba06fd19fba4d4c501073bd104f508eddba0..13fe718fefb17095f3b91a3efb59020210a6d779 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2687,6 +2687,11 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
       MainThreadScrollingReason::kNotScrollingOnMain;
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBegin");
 
+  // On Mac a scroll begin with |inertial_phase| = true happens to handle a
+  // fling.
+  if (scroll_state->is_in_inertial_phase())
+    return FlingScrollBegin();
+
   ClearCurrentlyScrollingLayer();
 
   gfx::Point viewport_point(scroll_state->position_x(),
@@ -3234,7 +3239,12 @@ void LayerTreeHostImpl::ScrollEnd(ScrollState* scroll_state) {
 
   DistributeScrollDelta(scroll_state);
   top_controls_manager_->ScrollEnd();
-  ClearCurrentlyScrollingLayer();
+
+  if (scroll_state->is_in_inertial_phase()) {

[dtapuska, 2016/08/18 00:30:45]

I'm a bit concerned this isn't correct for devices that don't have inertial
phase information or when the feature is off. Shouldnt it be clearing the
scrolling layer in those cases?

[sahel, 2016/08/18 15:41:56]

It's ok if we don't do the latching for whatever reason and don't clear it here,
because in scrollBegin we clear it.

+    // Only clear the currently scrolling layer if we know the scroll is done.
+    // A non-inertial scroll end could be followed by an inertial scroll.
+    ClearCurrentlyScrollingLayer();
+  }
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::FlingScrollBegin() {