RenderLayerCpmpositor has: if (updateType == CompositingUpdateType::OnScroll || updateType == CompositingUpdateType::OnCompositedScroll) { // Scrolling can affect overlap. FIXME: avoid for page scrolling. updateRoot->setDescendantsNeedCompositingRequirementsTraversal(); } We already extend overlap rects for fixed and sticky layers, so we should be able to avoid any new overlap testing on page scroll.