Don't repaint squashed, fixed position layers during scrolling

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1251 matching lines @@
 
     if (!m_viewportConstrainedObjects || m_viewportConstrainedObjects->isEmpty()) {
         hostWindow()->scroll();
         return true;
     }
 
     Region regionToUpdate;
     ViewportConstrainedObjectSet::const_iterator end = m_viewportConstrainedObjects->end();
     for (ViewportConstrainedObjectSet::const_iterator it = m_viewportConstrainedObjects->begin(); it != end; ++it) {
         RenderObject* renderer = *it;
-        // m_viewportConstrainedObjects should not contain non-viewport constrained objects.
         ASSERT(renderer->style()->hasViewportConstrainedPosition());
-
-        // Fixed items should always have layers.
         ASSERT(renderer->hasLayer());
         RenderLayer* layer = toRenderBoxModelObject(renderer)->layer();
 
-        // Layers that paint into their ancestor or into a grouped backing will still need
-        // to apply a paint invalidation. If the layer paints into its own backing, then
-        // it does not need paint invalidation just to scroll.
-        if (layer->compositingState() == PaintsIntoOwnBacking)
+        CompositingState state = layer->compositingState();
+        if (state == PaintsIntoOwnBacking || state == PaintsIntoGroupedBacking)
             continue;
 
         if (layer->hasAncestorWithFilterOutsets()) {
             // If the fixed layer has a blur/drop-shadow filter applied on at least one of its parents, we cannot
             // scroll using the fast path, otherwise the outsets of the filter will be moved around the page.
             return false;
         }
 
         IntRect updateRect = pixelSnappedIntRect(layer->paintInvalidator().paintInvalidationRectIncludingNonCompositingDescendants());
 
@@ skipping 1841 matching lines @@
 void FrameView::willRemoveScrollbar(Scrollbar* scrollbar, ScrollbarOrientation orientation)
 {
     ScrollableArea::willRemoveScrollbar(scrollbar, orientation);
     if (AXObjectCache* cache = axObjectCache()) {
         cache->remove(scrollbar);
         cache->handleScrollbarUpdate(this);
     }
 }
 
 } // namespace blink