Fable senior review (COMMENT, no blockers) + DRY nit:
- Widen the ResizeObserver from #msg to the whole #composerWrap so growth from the
attachment tray / selection chips (not just textarea rows) also re-pins a bottom-
pinned reader. Fable flagged tray/chip growth as a likely REMAINING #5515 trigger
— same viewport-shrink physics, now covered by the same guarded re-pin at one seam.
#composerWrap contains #msg, so the textarea path is still covered.
- DRY: _repinMessagesAfterComposerResize() now reuses _messageBottomDistance()
instead of re-implementing the bottom-distance expression inline.
Re-verified live (composer grow -> bottomDist stays 0) + 524 scroll/composer
regression tests green + 6 issue tests green.
7e7e71f695
