308027 2026-02-16 19:53:18 -0800 Dynamic slot insertion resets scroll 2026-04-03 18:04:31 -0700 1 1 1 Unclassified WebKit Layout and Rendering WebKit Nightly Build Unspecified Unspecified NEW InRadar P2 Normal --- 1 jacob webkit-unassigned bfulgham karlcow simon.fraser webkit-bug-importer zalan oldest_to_newest 2182270 0 478382 jacob 2026-02-16 19:53:18 -0800 Created attachment 478382 Simple reproduction test case In WebKit, inserting a <slot> dynamically inside a shadow DOM component that's also using position sticky can reset the main page scroll position when the component uses container-type: inline-size. This is reproducible in MiniBrowser (Release) and Safari/WebKit, and does not reproduce in Chromium/Firefox. Steps to Reproduce 1. Open the attached repro. 2. Scroll the page down (for example, ~900px) to the example shadow dom container. 3. Hover the target element in the shadow root. 4. On hover, JS appends a new <slot> into a shadow subtree. 5. if you are horizontally scrolled, as you moved your mouse when the slot is inserted, it will jump the scroll position back to the start of the container. 5. Observe page scroll position. Actual Results The scroll position is changing when slot is injected. Expected Results The scroll position should not be affected by slot injection. Build / Hardware - WebKit local build (Release), reproduced on macOS (Apple Silicon). - Reproduced in MiniBrowser built from current branch. 2182289 1 webkit-bug-importer 2026-02-16 21:28:37 -0800 <rdar://problem/170525902> 2196746 2 simon.fraser 2026-04-03 17:46:06 -0700 I see the scroll reset on the scroll container, but I don't see the page scrolling. 2196750 3 simon.fraser 2026-04-03 18:04:23 -0700 This happens when we do an interleaved layout, and layout the tree with most of the `EXAMPLE-CONTAINER` content missing; there's no overflow, so we scroll the overflow scroller to 0,0. Overflow is added back when we do the full layout later. The fix probably involves having interleaved layouts never call `RenderBlock::updateScrollInfoAfterLayout()`. 478382 2026-02-16 19:53:18 -0800 2026-02-16 19:53:18 -0800 Simple reproduction test case repro-2.html text/html 3618 jacob PCFkb2N0eXBlIGh0bWw+CjxodG1sIGxhbmc9ImVuIj4KPGhlYWQ+CjxtZXRhIGNoYXJzZXQ9InV0 Zi04Ij4KPG1ldGEgbmFtZT0idmlld3BvcnQiIGNvbnRlbnQ9IndpZHRoPWRldmljZS13aWR0aCwg aW5pdGlhbC1zY2FsZT0xIj4KPHRpdGxlPnJlcHJvLTI8L3RpdGxlPgo8c3R5bGU+CiAgICBib2R5 IHsKICAgICAgICBtYXJnaW46IDA7CiAgICAgICAgZm9udC1mYW1pbHk6IC1hcHBsZS1zeXN0ZW0s IEJsaW5rTWFjU3lzdGVtRm9udCwgIlNlZ29lIFVJIiwgc2Fucy1zZXJpZjsKICAgIH0KCiAgICAu c3BhY2VyIHsKICAgICAgICBoZWlnaHQ6IDEyMDBweDsKICAgIH0KCiAgICAjbW91bnQgewogICAg ICAgIHdpZHRoOiBtaW4oOTJ2dywgNjQwcHgpOwogICAgICAgIG1hcmdpbjogMCBhdXRvOwogICAg fQo8L3N0eWxlPgo8L2hlYWQ+Cjxib2R5Pgo8ZGl2IGNsYXNzPSJzcGFjZXIiPjwvZGl2Pgo8ZGl2 IGlkPSJtb3VudCI+PC9kaXY+CjxkaXYgY2xhc3M9InNwYWNlciI+PC9kaXY+Cgo8c2NyaXB0Pgpj 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