Land layer squashing behind a flag

Source/core/rendering/RenderObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1379 matching lines @@
     return repaintContainer;
 }
 
 void RenderObject::repaintUsingContainer(const RenderLayerModelObject* repaintContainer, const IntRect& r) const
 {
     if (!repaintContainer) {
         view()->repaintViewRectangle(r);
         return;
     }
 
+    if (repaintContainer->compositingState() == PaintsIntoGroupedBacking) {
+        // FIXME: this if-statement may not catch the repaint invalidation that comes from a descendant that contributes to this squashed subtree.
+
+        ASSERT(repaintContainer->groupedMapping());
+
+        // Not clean, but if squashing layer does not yet exist here (e.g. repaint invalidation coming from within recomputing compositing requirements)
+        // then it's ok to just exit here, since the squashing layer will get repainted when it is newly created.
+        if (!repaintContainer->groupedMapping()->squashingLayer())
+            return;
+
+        IntRect offsetRect(r);
+        offsetRect.move(-repaintContainer->layer()->offsetFromSquashingLayerOrigin());
+        repaintContainer->groupedMapping()->squashingLayer()->setNeedsDisplayInRect(offsetRect);
+        return;
+    }
+
     if (repaintContainer->isRenderFlowThread()) {
         toRenderFlowThread(repaintContainer)->repaintRectangleInRegions(r);
         return;
     }
 
     if (repaintContainer->hasFilter() && repaintContainer->layer()->requiresFullLayerImageForFilters()) {
         repaintContainer->layer()->repainter().setFilterBackendNeedsRepaintingInRect(r);
         return;
     }
 
@@ skipping 1946 matching lines @@
 {
     if (object1) {
         const WebCore::RenderObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showRenderTreeAndMark(object1, "*", object2, "-", 0);
     }
 }
 
 #endif