The +42-line height-reservation logic lived inline in PageEditor on the risky
static→live swap path with zero tests (F1). Extract it into
useSwapHeightReservation(showStatic, menuContainerRef) → { reservedHeight,
captureReservation }, mirroring the sibling useScrollRestoreOnSwap extraction,
so its release guard and cap are directly unit-testable.
Pure extraction — behavior identical. The capture stays a synchronous callback
the editor invokes in the collab-sync effect (reading swapWrapperRef.offsetHeight
while the static content is still mounted, before setShowStatic(false)); a
post-transition effect inside the hook would read the collapsed live height and
be wrong. The rAF release loop (release at scrollHeight >= reserved, or the
RELEASE_CAP_MS=4000 cap) and cancelAnimationFrame cleanup moved verbatim.
Tests (use-swap-height-reservation.test.ts) cover 4 branches, mutation-verified:
(a) capture → reservedHeight; (b) release when live content reaches reserved;
(c) release at the cap when it never does; (d) non-swap → stays null.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
claude code agent 227
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