Fix squashing to treat perspective and preserve3d elements as transform ancestors.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

Index: Source/core/rendering/compositing/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.cpp b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
index 1924a3268a64f8c9bdbe32b0526ef477db51102e..426170eb85de7009db385f305ed198a1f40d0d6f 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -54,6 +54,7 @@
 #include "platform/LengthFunctions.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/fonts/FontCache.h"
+#include "platform/geometry/TransformState.h"
 #include "platform/graphics/GraphicsContext.h"
 #include "wtf/CurrentTime.h"
 #include "wtf/text/StringBuilder.h"
@@ -761,11 +762,12 @@ void CompositedLayerMapping::updateOverflowControlsHostLayerGeometry(const Rende
 
             m_overflowControlsHostLayer->setPosition(IntPoint(-m_overflowControlsClippingLayer->offsetFromRenderer()));
         } else {
-            ASSERT(m_owningLayer.transformAncestor() == compositingStackingContext->transformAncestor());
-            LayoutPoint localOffsetToTransformedAncestor = m_owningLayer.computeOffsetFromTransformedAncestor();
-            LayoutPoint compositingStackingContextOffsetToTransformedAncestor = compositingStackingContext->computeOffsetFromTransformedAncestor();
-
-            m_overflowControlsHostLayer->setPosition(FloatPoint(localOffsetToTransformedAncestor - compositingStackingContextOffsetToTransformedAncestor));
+            // The controls are in the same 2D space as the compositing container, so we can map them into the space of the container.
+            TransformState transformState(TransformState::ApplyTransformDirection, FloatPoint());
+            m_owningLayer.renderer()->mapLocalToContainer(compositingStackingContext->renderer(), transformState, ApplyContainerFlip);
+            transformState.flatten();
+            LayoutPoint offsetFromStackingContainer = LayoutPoint(transformState.lastPlanarPoint());
+            m_overflowControlsHostLayer->setPosition(FloatPoint(offsetFromStackingContainer));
         }
     } else {
         m_overflowControlsHostLayer->setPosition(FloatPoint());