Bug 292972

Created attachment 475222 [details] Test case See test case. Test case has a scroll driven animation, a button that triggers a popover element that's anchor-positioned to the button. When the popover is shown, it keeps getting painted over and over again, about 1 per frame. Enable "Show layer borders" to see the paint counter going up and up. Root cause: AnchorPositionEvaluator::updateAnchorPositioningStatesAfterInterleavedLayout() calls setNeedsLayout() once on each anchor-positioned element. setNeedsLayout() repaints the entire layer if the element is in a layer: inline void RenderObject::setNeedsLayout(MarkingBehavior markParents) { [...] if (hasLayer()) setLayerNeedsFullRepaint(); } Since anchor-positioned element is a popover, it's placed in the top layer. Hence it has a layer and the layer is forced to repaint. Because AnchorPositionEvaluator::updateAnchorPositioningStatesAfterInterleavedLayout() is called at least once during style/layout interleaving, this guarantees the layer is repainted at least once per style resolution. When there's an animation, the style is resolved every frame, so the effect is that the layer is repainted every frame. Ideally, we don't setNeedsLayout() on each anchor-positioned element every style resolution. It should only be called when the anchor the element depends on changes (in size or position) to re-layout the anchor-positioned element. This should be done in Style::Scope::invalidateForAnchorDependencies.