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 dbe5358b397061d3e7231b524cf1a61e6217cbaf..512ff4dbe61111e0232efa6562a9d2a4880bd48e 100644
--- a/Source/core/rendering/RenderObject.cpp
+++ b/Source/core/rendering/RenderObject.cpp
@@ -1397,6 +1397,34 @@ void RenderObject::repaintUsingContainer(const RenderLayerModelObject* repaintCo
         return;
     }
 
+    if (repaintContainer->compositingState() == PaintsIntoGroupedBacking) {
+        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);
+
+        // First, convert the repaint rect into the space of the repaintContainer
+        TransformState transformState(TransformState::ApplyTransformDirection, FloatQuad(FloatRect(r)));
+        mapLocalToContainer(repaintContainer, transformState, ApplyContainerFlip);
+        transformState.flatten();
+        offsetRect = transformState.lastPlanarQuad().enclosingBoundingBox();
+
+        // FIXME: the repaint rect computed below could be tighter in uncommon nested transform cases, if we passed the quad
+        // directly to the next chunk of code.
+
+        // Then, convert the repaint rect from repaintConainer space into the squashing GraphicsLayer's coordinates.
+        if (repaintContainer->hasTransform())
+            offsetRect = repaintContainer->layer()->transform()->mapRect(r);
+        offsetRect.move(-repaintContainer->layer()->offsetFromSquashingLayerOrigin());
+        repaintContainer->groupedMapping()->squashingLayer()->setNeedsDisplayInRect(offsetRect);
+        return;
+    }
+
     if (repaintContainer->isRenderFlowThread()) {
         toRenderFlowThread(repaintContainer)->repaintRectangleInRegions(r);
         return;