Land layer squashing behind a flag

Source/core/rendering/RenderObject.cpp

Index: Source/core/rendering/RenderObject.cpp
diff --git a/Source/core/rendering/RenderObject.cpp b/Source/core/rendering/RenderObject.cpp
index 27896d5978ef96f3e9518c672c7e79dda38c75e2..44f3d571f08b6b52529df12aa52d20d52b4cc953 100644
--- a/Source/core/rendering/RenderObject.cpp
+++ b/Source/core/rendering/RenderObject.cpp
@@ -1397,6 +1397,22 @@ void RenderObject::repaintUsingContainer(const RenderLayerModelObject* repaintCo
         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;