292972 2025-05-13 23:39:11 -0700 [css-anchor-position-1] animation + anchor-positioned element as popover cause anchor-positioned element to be repainted every style resolution 2025-05-19 09:45:59 -0700 1 1 1 Unclassified WebKit CSS WebKit Nightly Build Unspecified Unspecified NEW InRadar P2 Normal --- 291856 1 kiet.ho kiet.ho fantasai.bugs kiet.ho koivisto webkit-bug-importer oldest_to_newest 2116965 0 475222 kiet.ho 2025-05-13 23:39:11 -0700 Created attachment 475222 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. 2116966 1 webkit-bug-importer 2025-05-13 23:39:19 -0700 <rdar://problem/151285174> 475222 2025-05-13 23:39:11 -0700 2025-05-13 23:39:11 -0700 Test case index.html text/html 975 kiet.ho PCFET0NUWVBFIGh0bWw+CjxodG1sPgoKPGhlYWQ+CiAgPHN0eWxlPgogICAgQGtleWZyYW1lcyBn cm93LXByb2dyZXNzIHsKICAgICAgZnJvbSB7CiAgICAgICAgdHJhbnNmb3JtOiBzY2FsZVgoMCk7 CiAgICAgIH0KCiAgICAgIHRvIHsKICAgICAgICB0cmFuc2Zvcm06IHNjYWxlWCgxKTsKICAgICAg fQogICAgfQoKICAgIGZvb3Rlcjo6YWZ0ZXIgewogICAgICBjb250ZW50OiAiIjsKICAgICAgaGVp Z2h0OiAxZW07CiAgICAgIHdpZHRoOiAxMDAlOwogICAgICBiYWNrZ3JvdW5kOiB5ZWxsb3c7CiAg ICAgIGxlZnQ6IDA7CiAgICAgIGJvdHRvbTogMDsKICAgICAgcG9zaXRpb246IGZpeGVkOwogICAg ICB0cmFuc2Zvcm0tb3JpZ2luOiB0b3AgbGVmdDsKCiAgICAgIGFuaW1hdGlvbjogZ3Jvdy1wcm9n cmVzcyBsaW5lYXI7CiAgICAgIGFuaW1hdGlvbi10aW1lbGluZTogc2Nyb2xsKCk7CiAgICB9Cgog ICAgLnByb2ZpbGUtYnV0dG9uIHsKICAgICAgYW5jaG9yLW5hbWU6IC0tcHJvZmlsZS1idXR0b247 CiAgICAgIHdpZHRoOiAxMDBweDsKICAgICAgaGVpZ2h0OiAxMDBweDsKICAgIH0KCiAgICAucHJv ZmlsZS1tZW51IHsKICAgICAgbWFyZ2luOiAwOwoKICAgICAgd2lkdGg6IDEwMHB4OwogICAgICBo ZWlnaHQ6IDEwMHB4OwogICAgICBiYWNrZ3JvdW5kOiBibGFjazsKCiAgICAgIHBvc2l0aW9uLWFu Y2hvcjogLS1wcm9maWxlLWJ1dHRvbjsKICAgICAgcG9zaXRpb24tYXJlYTogYm90dG9tIHJpZ2h0 OwogICAgICBwb3NpdGlvbjogYWJzb2x1dGU7CiAgICB9CiAgPC9zdHlsZT4KPC9oZWFkPgoKPGJv ZHk+CiAgPGJ1dHRvbiBwb3BvdmVydGFyZ2V0PSJwcm9maWxlLW1lbnUiIGNsYXNzPSJwcm9maWxl LWJ1dHRvbiI+QlVUVE9OPC9idXR0b24+CiAgPHVsIGNsYXNzPSJwcm9maWxlLW1lbnUiIHBvcG92 ZXIgaWQ9InByb2ZpbGUtbWVudSI+PC91bD4KICA8Zm9vdGVyPjwvZm9vdGVyPgo8L2JvZHk+Cgo8 L2h0bWw+