Don't update graphics layer positions during coordinated scrolling

Source/core/rendering/RenderLayerCompositor.cpp

 /*
  * Copyright (C) 2009, 2010 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 1136 matching lines @@
 }
 
 void RenderLayerCompositor::frameViewDidScroll()
 {
     FrameView* frameView = m_renderView->frameView();
     IntPoint scrollPosition = frameView->scrollPosition();
 
     if (!m_scrollLayer)
         return;
 
-    // If there's a scrolling coordinator that manages scrolling for this frame view,
-    // it will also manage updating the scroll layer position.
+    bool scrollingCoordinatorHandlesOffset = false;
     if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator()) {
         if (Settings* settings = m_renderView->document()->settings()) {
             if (isMainFrame() || settings->compositedScrollingForFramesEnabled())
-                scrollingCoordinator->scrollableAreaScrollLayerDidChange(frameView);
+                scrollingCoordinatorHandlesOffset = scrollingCoordinator->scrollableAreaScrollLayerDidChange(frameView);
         }
     }
 
-    m_scrollLayer->setPosition(FloatPoint(-scrollPosition.x(), -scrollPosition.y()));
+#ifndef BLINK_SCROLLING_POSITION_NO_OFFSET
+    // FIXME: Remove when possible. Only required to stage multi-repo change.
+    scrollingCoordinatorHandlesOffset = false;
+#endif
+
+    // Scroll position = scroll minimum + scroll offset. Adjust the layer's
+    // position to handle whatever the scroll coordinator isn't handling.
+    // The minimum scroll position is non-zero for RTL pages with overflow.
+    if (scrollingCoordinatorHandlesOffset)
+        m_scrollLayer->setPosition(-frameView->minimumScrollPosition());
+    else
+        m_scrollLayer->setPosition(-scrollPosition);
 }
 
 void RenderLayerCompositor::frameViewDidLayout()
 {
 }
 
 void RenderLayerCompositor::rootFixedBackgroundsChanged()
 {
     if (!supportsFixedRootBackgroundCompositing())
         return;
 
     // To avoid having to make the fixed root background layer fixed positioned to
     // stay put, we position it in the layer tree as follows:
     //
     // + Overflow controls host
     //   + Frame clip
     //     + (Fixed root background) <-- Here.
     //     + Frame scroll
     //       + Root content layer
     //   + Scrollbars
     //
     // That is, it needs to be the first child of the frame clip, the sibling of
     // the frame scroll layer. The compositor does not own the background layer, it
     // just positions it (like the foreground layer).
     if (GraphicsLayer* backgroundLayer = fixedRootBackgroundLayer())
         m_clipLayer->addChildBelow(backgroundLayer, m_scrollLayer.get());
 }
 
-void RenderLayerCompositor::scrollingLayerDidChange(RenderLayer* layer)
+bool RenderLayerCompositor::scrollingLayerDidChange(RenderLayer* layer)
 {
     if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator())
-        scrollingCoordinator->scrollableAreaScrollLayerDidChange(layer);
+        return scrollingCoordinator->scrollableAreaScrollLayerDidChange(layer);
+    return false;
 }
 
 String RenderLayerCompositor::layerTreeAsText(LayerTreeFlags flags)
 {
     updateCompositingLayers(CompositingUpdateAfterLayout);
 
     if (!m_rootContentLayer)
         return String();
 
     // We skip dumping the scroll and clip layers to keep layerTreeAsText output
@@ skipping 1522 matching lines @@
     info.addMember(m_layerForScrollCorner, "layerForScrollCorner");
 #if ENABLE(RUBBER_BANDING)
     info.addMember(m_layerForOverhangAreas, "layerForOverhangAreas");
     info.addMember(m_contentShadowLayer, "contentShadowLayer");
     info.addMember(m_layerForTopOverhangArea, "layerForTopOverhangArea");
     info.addMember(m_layerForBottomOverhangArea, "layerForBottomOverhangArea");
 #endif
 }
 
 } // namespace WebCore