- Shipped
- August 20, 2026 at 10:18 PM UTC
- Author
- Kamo
- Commit
- 861e159
The nav's scrolled/at-top state came from a scroll handler that read window.scrollY, and the mount effect called that handler synchronously during React's commit phase. Reading a scroll offset there forces the browser to compute layout on the spot, before the first paint has happened, which is precisely the forced reflow Lighthouse was reporting against the router chunk. An IntersectionObserver on a 1x20px sentinel at the top of the document answers the same question — is the page scrolled past 20px — without asking for geometry at all. The browser reports it out of a pass it was already going to do. The sentinel is absolutely positioned and pointer-events: none, so it is out of flow and cannot shift or intercept anything.