Issue repaints before and after changing the squashing assignment of a RenderLayer.

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 a39e94c175a6b0333a59eb6ed89eb8b6f9974306..8f63184303482ada56d28a7ef616b994e2ee7867 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -2182,10 +2182,12 @@ bool CompositedLayerMapping::updateSquashingLayerAssignment(RenderLayer* squashe
     bool updatedAssignment = false;
     if (nextSquashedLayerIndex < m_squashedLayers.size()) {
         if (!paintInfo.isEquivalentForSquashing(m_squashedLayers[nextSquashedLayerIndex])) {
+            compositor()->repaintOnCompositingChange(squashedLayer);
             updatedAssignment = true;
             m_squashedLayers[nextSquashedLayerIndex] = paintInfo;
         }
     } else {
+        compositor()->repaintOnCompositingChange(squashedLayer);
         m_squashedLayers.append(paintInfo);
         updatedAssignment = true;
     }