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npub1cc3ha7z055mu0rwwu7806t2wt8mj3pvu0uv5mfp2c50dahaqhczshdalg6andTyler Longwell 963133b6a9 feat(desktop): wire measured column width into row estimates + anchor-contract guard

Two of the deferred scroll-jitter seams, on top of the integrated fix train
(T1.1 gate -> T2 writer -> T3 estimator @ 1fa4551a):

- columnWidthPx: measure the timeline row column once (useElementWidth) and
  thread it into timelineRowReserveStyle so row-height reserves use the real
  wrap width instead of the 64-char fallback. Pre-measure zero passes undefined
  to preserve the estimator's own fallback rather than tripping its min-chars
  floor. On jitter-corpus this shrinks estimate-error R from 75 -> 55 peak
  (better reserves = smaller realized-vs-reserved delta) while the motion gate
  stays GREEN 0.00/0.00 -- R moves only in the estimator lane, the gate only in
  the writer lane, cross-lane invariant intact.

- Wren #4 anchor-contract guard: a companion perf test that reads the PRODUCTION
  computed overflow-anchor on the real conversation scroller (no test override)
  and asserts it resolves to none. The jitter gate forces the property for its
  measurement; this proves the shipped stylesheet actually ships it, so a
  regression that hands correction back to Chromium's native anchoring (masking
  a WKWebView-only on-device break) fails loudly.

The onRealizedHeight seam is intentionally NOT wired: T4 ground truth
(RESEARCH/GUI_SCROLL_RECLAIM_T4.md) shows the upscroll path is append-only with
no unmount, so it needs no JS height cache; a per-event-id cache is only the
deferred, narrowly-scoped T4b for the head-refetch case. Adding an unused
callback now would be dead abstraction.

Validation:
- pnpm --dir desktop typecheck: clean
- biome check on both changed files: clean
- rowHeightEstimate.test.mjs 16/16, useAnchoredScroll.test.mjs 2/2
- perf: upscroll-jitter GREEN 0.00/0.00 (R 55), anchor-contract PASS

Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>

2026-07-08 14:43:41 -04:00