The tablet icon-rail collapse is mounted-gated (SSR and the hydration
frame paint the expanded 256px shell to avoid a React #418 mismatch),
so the resolved layout lands on the first client render after hydration
settles. On a loaded runner that can arrive just after networkidle,
letting the guard measure a mid-hydration frame (main=512 instead of
~704). Wait for the content column to reach its settled width before
measuring; a real regression still fails via the wait timing out.
Marc-André Bombeck