Pass integer scroll position to CC if frameView has non-layered viewport constrained objects

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 1bfd1b4189819e4f7894d9705a92eafd8dfe18a5..edf1aa01487206103aa285aedebb9291f6c2325f 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -355,7 +355,7 @@ void ScrollingCoordinator::scrollableAreaScrollbarLayerDidChange(ScrollableArea*
         removeWebScrollbarLayer(scrollableArea, orientation);
 }
 
-bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* scrollableArea)
+bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* scrollableArea, bool hasVisibleViewportConstrainedObjects)
 {
     GraphicsLayer* scrollLayer = scrollableArea->layerForScrolling();
 
@@ -368,7 +368,20 @@ bool ScrollingCoordinator::scrollableAreaScrollLayerDidChange(ScrollableArea* sc
     WebLayer* containerLayer = toWebLayer(scrollableArea->layerForContainer());
     if (webLayer) {
         webLayer->setScrollClipLayer(containerLayer);
-        webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition()));
+        // Viewport constrained objects, e.g. fixed position elements, are
+        // positioned in Blink using integer coordinates. In that case, we
+        // don't want to set the WebLayer's scroll position at fractional
+        // precision otherwise the WebLayer's position after snapping to
+        // device pixel can be off with regard to fixed position elements.
+        // FIXME: fixed position elements which have their own compositing
+        // layer can actually be positioned at device pixel boundary in
+        // chromium compositor, so this should really be conditioned on
+        // non-composited fixed position elements. crbug.com/414283.
+        if (hasVisibleViewportConstrainedObjects) {

[Rick Byers, 2014/12/05 19:01:41]

Rather than taking this bit in from our caller (which computes it independently
in FrameView), can we just test m_lastMainThreadScrollingReasons for the
HasNonLayerViewportConstrainedObjects flag?  We really should be able to share
this logic somehow.  I don't know if there are potential timing issues where
m_lastMainThreadScrollingReasons is out of date.

+            webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPosition() - scrollableArea->minimumScrollPosition()));
+        } else {
+            webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition()));
+        }
         webLayer->setBounds(scrollableArea->contentsSize());
         bool canScrollX = scrollableArea->userInputScrollable(HorizontalScrollbar);
         bool canScrollY = scrollableArea->userInputScrollable(VerticalScrollbar);