Plumb an overscroll callback form the compositor to the Android UI thread.

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index 42aa9533c928df6b108bf063cb76bb91c546d130..75a733be6f0db701b0b4bfdebe85d042b6386f4c 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -769,6 +769,8 @@ CompositorFrameMetadata LayerTreeHostImpl::makeCompositorFrameMetadata() const
     metadata.root_scroll_offset = rootScrollLayer()->scrollOffset() + rootScrollLayer()->scrollDelta();
     if (!m_settings.pageScalePinchZoomEnabled)
         metadata.root_scroll_offset.Scale(1 / m_pinchZoomViewport.page_scale_factor());
+    metadata.unused_scroll_delta = m_pendingScrollDelta;
+    m_pendingScrollDelta = gfx::Vector2dF();

[mkosiba (inactive), 2013/01/30 14:46:23]

where would be a good place to reset is variable? I tried doing that in
OnSendFrameToParentCompositorAck, didDrawAllLayers and processScrollDeltas but
all of those seem to happen too early.

[aelias_OOO_until_Jul13, 2013/01/30 18:02:48]

I guess it's okay to do it here.  You can remove the const from the method.

 
     return metadata;
 }
@@ -808,6 +810,7 @@ void LayerTreeHostImpl::didDrawAllLayers(const FrameData& frame)
     for (size_t i = 0; i < frame.willDrawLayers.size(); ++i)
         frame.willDrawLayers[i]->didDraw(m_resourceProvider.get());
 
+
     // Once all layers have been drawn, pending texture uploads should no
     // longer block future uploads.
     m_resourceProvider->markPendingUploadsAsNonBlocking();
@@ -1323,6 +1326,7 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
         return false;
 
     gfx::Vector2dF pendingDelta = scrollDelta;
+    gfx::Vector2dF overScrollDelta = scrollDelta;

[aelias_OOO_until_Jul13, 2013/01/30 18:02:48]

Call this "unused" for consistency.

     bool didScroll = false;
 
     for (LayerImpl* layerImpl = currentlyScrollingLayer(); layerImpl; layerImpl = layerImpl->parent()) {
@@ -1344,6 +1348,7 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
         if (appliedDelta.LengthSquared() < moveThresholdSquared)
             continue;
         didScroll = true;
+        overScrollDelta.Subtract(appliedDelta);
 
         // If the applied delta is within 45 degrees of the input delta, bail out to make it easier
         // to scroll just one layer in one direction without affecting any of its parents.
@@ -1362,11 +1367,14 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
             break;
     }
 
-    if (didScroll) {
+    m_pendingScrollDelta += overScrollDelta;
+
+    if (didScroll || !gfx::ToFlooredVector2d(overScrollDelta).IsZero()) {

[mkosiba (inactive), 2013/01/30 14:46:23]

this makes us draw frames even though the contents of those frames would not
change. is there a simple change I could do to optimize that away or is it not
worth it?

[aelias_OOO_until_Jul13, 2013/01/30 18:02:48]

I think the redraw itself is okay but let's avoid the commit and tree priority
update, you shouldn't need those.

         m_client->setNeedsCommitOnImplThread();
         m_client->setNeedsRedrawOnImplThread();
         m_client->renewTreePriority();
     }
+
     return didScroll;
 }