[flamboh]

Fix for the redesign stack, targets layer 1 (#28839).

Note

🤖 GPT-6.1 Sol on behalf of Oliver

Opening a thread at its end can briefly show older rows. Sending can leave the new user message under the composer until a later scroll correction.

The timeline now corrects these positions from measured content in observeResize, before paint. Opening corrections stop with the existing restoration operation. Send corrections apply only when a new user row arrives in the same thread while follow is enabled; they preserve an active turn anchor and leave ordinary streamed row growth alone.

The affected code originated in layer 1. This uses the before-paint idea from #31450, but watches content during restoration and user-message insertion rather than viewport resizing. It does not change the history-collapse measurement in #31272, the footer in #31401, or the streaming-follow behavior in #31521.

Validation

• Failing-first browser regressions on layer 1: opening a 25-row thread initially leaves a 5,054 px end gap; an unanchored send initially overlaps composer clearance by 78.75 px. The corrected first measured frame meets both assertions. Sends with an active anchor are covered too.
• The full MessagesTimeline.browser.test.tsx suite passes: 43 tests, including saved mid-thread positions and find navigation.
• Integrated web client at 1280 × 800, isolated Replay showcase history: the first before-paint send sample has scrollTop=1295, end gap 0, message bounds 430–520.75, height 90.75, and composer top 616.5. No paid provider turns. The integrated thread was shorter than the 25-row regression fixture.
• vp run check:changed origin/maria/redesign/1-primitives-shell passes: format, lint, both knip checks, affected typechecks, 137 tests across four files, and two React Compiler checks.

Before / after

Captures pending.

GPT-6.1 Sol in the Codex harness (T3 Code).