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

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 350 matching lines @@
 
     if (scrollLayer) {
         ASSERT(m_page);
         scrollLayer->setScrollableArea(scrollableArea, isForViewport(scrollableArea));
     }
 
     WebLayer* webLayer = toWebLayer(scrollableArea->layerForScrolling());
     WebLayer* containerLayer = toWebLayer(scrollableArea->layerForContainer());
     if (webLayer) {
         webLayer->setScrollClipLayer(containerLayer);
-        webLayer->setScrollPositionDouble(DoublePoint(scrollableArea->scrollPositionDouble() - scrollableArea->minimumScrollPosition()));
+        // Non-layered 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.
+        if (m_lastMainThreadScrollingReasons & ScrollingCoordinator::HasNonLayerViewportConstrainedObjects)
+            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);
         webLayer->setUserScrollable(canScrollX, canScrollY);
     }
     if (WebScrollbarLayer* scrollbarLayer = getWebScrollbarLayer(scrollableArea, HorizontalScrollbar)) {
         GraphicsLayer* horizontalScrollbarLayer = scrollableArea->layerForHorizontalScrollbar();
         if (horizontalScrollbarLayer)
             setupScrollbarLayer(horizontalScrollbarLayer, scrollbarLayer, webLayer, containerLayer);
     }
@@ skipping 154 matching lines @@
     for (const auto& scrollbar : m_horizontalScrollbars)
         GraphicsLayer::unregisterContentsLayer(scrollbar.value->layer());
     for (const auto& scrollbar : m_verticalScrollbars)
         GraphicsLayer::unregisterContentsLayer(scrollbar.value->layer());
 
     m_horizontalScrollbars.clear();
     m_verticalScrollbars.clear();
     m_layersWithTouchRects.clear();
     m_wasFrameScrollable = false;
 
-    // This is retained for testing.
     m_lastMainThreadScrollingReasons = 0;
     setShouldUpdateScrollLayerPositionOnMainThread(m_lastMainThreadScrollingReasons);
 }
 
 // Note that in principle this could be called more often than computeTouchEventTargetRects, for
 // example during a non-composited scroll (although that's not yet implemented - crbug.com/261307).
 void ScrollingCoordinator::setTouchEventTargetRects(LayerHitTestRects& layerRects)
 {
     TRACE_EVENT0("input", "ScrollingCoordinator::setTouchEventTargetRects");
 
@@ skipping 433 matching lines @@
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : nullptr)
         return blink::WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink