When performing a fast-path scroll, issue repaints for viewport-constrained layers

Source/core/frame/FrameView.cpp

Index: Source/core/frame/FrameView.cpp
diff --git a/Source/core/frame/FrameView.cpp b/Source/core/frame/FrameView.cpp
index 69e42ef47414032d9ece4f360069cc4aa9996db5..0a043b360750f4097852dc8cbf5a87af31cbe8e0 100644
--- a/Source/core/frame/FrameView.cpp
+++ b/Source/core/frame/FrameView.cpp
@@ -1419,8 +1419,8 @@ bool FrameView::scrollContentsFastPath(const IntSize& scrollDelta, const IntRect
 
         IntRect updateRect = pixelSnappedIntRect(layer->repainter().repaintRectIncludingNonCompositingDescendants());
 
-        RenderLayer* enclosingCompositingLayer = layer->enclosingCompositingLayer(ExcludeSelf);
-        if (enclosingCompositingLayer && !enclosingCompositingLayer->renderer()->isRenderView()) {
+        const RenderLayerModelObject* repaintContainer = layer->renderer()->containerForRepaint();
+        if (repaintContainer && !repaintContainer->isRenderView()) {
             // If the fixed-position layer is contained by a composited layer that is not its containing block,
             // then we have to invalidate that enclosing layer, not the RenderView.
             // FIXME: Why do we need to issue this invalidation? Won't the fixed position element just scroll
@@ -1429,7 +1429,7 @@ bool FrameView::scrollContentsFastPath(const IntSize& scrollDelta, const IntRect
             IntRect previousRect = updateRect;
             previousRect.move(scrollDelta);
             updateRect.unite(previousRect);
-            enclosingCompositingLayer->repainter().setBackingNeedsRepaintInRect(updateRect);
+            layer->renderer()->repaintUsingContainer(repaintContainer, updateRect, InvalidationScroll);
         } else {
             // Coalesce the paint invalidations that will be issued to the renderView.
             updateRect = contentsToRootView(updateRect);