Fix one-time content jump when reactivating a room at the bottom#904
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On a room switch MessageList remounts and the virtualizer is seeded from the persistent row-height cache. @TanStack measures rows via ResizeObserver; on WebKit the settled measurement is delivered late (after the row is windowed out or the list unmounts), so the cache keeps the row's transient pre-settle height. The next entry seeds that wrong height, the visible rows reflow to their real height after the first paint, and the bottom-pin re-asserts — a one-time content jump (visible on WebKit, smooth on Chromium). Snapshot each mounted row's settled offsetHeight on unmount and write it back to the cache, overriding the transient value, so the next seed matches the render and there is no post-paint reflow.
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Switching away from a room and back (at the bottom) caused the message list to visibly jump once on Tauri/WebKit.
Cause:
MessageListremounts per room and the virtualizer is seeded from the persistent row-height cache. @TanStack measures rows viaResizeObserver; on WebKit the settled measurement is delivered late — after the row is windowed out or the list unmounts — so the cache keeps the row's transient pre-settle height. The next entry seeds that wrong height, the visible rows reflow to their real height after the first paint, and the bottom-pin re-asserts. Chromium repaints this smoothly (invisible); WebKit shows it as a one-time jump, every switch.Fix: snapshot each mounted row's settled
offsetHeighton unmount and write it back to the cache, overriding the transient value. The next seed then matches the render exactly, so there is no post-paint reflow and no re-pin.Additive — does not touch the pin loop. Verified via typecheck and the MessageList/height-cache unit tests. The visual fix is WebKit-specific and needs confirmation on
tauri:dev.