Phase 52's first pass shipped `position: sticky; bottom` on `.composer` and
called the phase done, but the owner's requirement — "the chat message-input
textarea should be at the bottom of the screen" — still failed in the browser:
on an empty/short chat the input rested just under the empty state (~57% of
the viewport) with a dead band down to the footer.
`position: sticky` can only pull a box UP toward the scrollport's bottom edge;
it can never push a box DOWN to meet it, so on a page that does not overflow
it is a no-op. The old story suite only exercised an overflowing conversation
(one test even asserted the buggy resting position as expected), which is why
the half-fix passed.
- `.messages { flex: 1 1 auto }` — absorbs a short page's free space so the
composer's resting in-flow position is the bottom of the full-height column
(body min-height:100dvh -> .app-main flex:1 -> .chat-shell flex:1); basis
stays `auto`, no height cap, no overflow — the document stays the scroller
- `.composer { bottom: env(safe-area-inset-bottom, 0) }` — the explicit 0
fallback replaces the env()-only offset, which degraded to `auto` (no pin)
wherever env() is unsupported
- E2E: `test_empty_chat_composer_sits_in_normal_flow` ->
`..._at_the_screen_bottom` (chrome-only band below the resting composer);
the phone suite now checks the resting position as well as the pinned one
- Unit pins: the flex-grow half and the full-height column are pinned, so the
fix cannot silently regress to sticky-only
Still CSS-only — no DOM change, no JS, no new scroll call site (phase 42
never-auto-scroll contract intact), no z-index.
Verified: 1019 unit/integration tests pass (app/ coverage 99%), ruff and
pyright clean; tests/e2e/test_pinned_composer.py green in isolation (4), plus
the stop/autoscroll/persistence/mobile-nav suites and 14 layout/scroll
neighbours green in isolation.
ducoterra
aba8615177