Avoid tree walks when computing RenderLayer::scrollParent

Source/core/rendering/compositing/RenderLayerCompositor.cpp

Index: Source/core/rendering/compositing/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/compositing/RenderLayerCompositor.cpp b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
index 32563e506ffc7042cd66ec0c7de38d3caf1fb4bb..f62b204885829561013f04195cf29e35b2e02f3a 100644
--- a/Source/core/rendering/compositing/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
@@ -388,6 +388,14 @@ void RenderLayerCompositor::setNeedsCompositingUpdate(CompositingUpdateType upda
     m_renderView.frameView()->scheduleAnimation();
 }
 
+static void clearAncestorDependentPropertyCacheRecursive(RenderLayer* layer)
+{
+    layer->clearAncestorDependentPropertyCache();
+    RenderLayerStackingNodeIterator iterator(*layer->stackingNode(), AllChildren);
+    for (RenderLayer* child = layer->firstChild(); child; child = child->nextSibling())
+        clearAncestorDependentPropertyCacheRecursive(child);
+}
+
 void RenderLayerCompositor::updateCompositingLayers()
 {
     TRACE_EVENT0("blink_rendering", "RenderLayerCompositor::updateCompositingLayers");
@@ -405,6 +413,9 @@ void RenderLayerCompositor::updateCompositingLayers()
 
     updateCompositingLayersInternal();
 
+    // Clear data only valid during compositing updates.
+    clearAncestorDependentPropertyCacheRecursive(rootRenderLayer());
+
     lifecycle().advanceTo(DocumentLifecycle::CompositingClean);
 
     DocumentAnimations::startPendingAnimations(m_renderView.document());