PageShell is reused (never remounted) across feed↔channel navigation — same
component position in App.tsx — so a one-shot useLayoutEffect kept its
ResizeObserver bound to the FIRST header node. On channel-open that node detached
and fired the observer with a 0 box, rewriting --hdr-h to 0+0+8=8px on the
still-mounted content div. Back restored the pt-(--hdr-h) pad class with that
stale value, sliding the content toolbar up under the header — stuck until reload.
Hold both the header and content nodes in state (callback refs) and key the
measurement effect to the live nodes, so it re-runs every time the header
mounts/unmounts; drop the stale pad when the header is absent (channel page).
Adds a regression test: header removed then restored must re-measure, not 8px.
peter
f6192aa6bf