fix(web): recover timeline following and bottom navigation

[bloom-street]

Problem

A timeline can fall away from its latest answer while automatic-follow intent remains armed. The original onIsAtEndChange guard hides the scroll-to-end button in that state, leaving no recovery affordance. New-turn framing also keeps a text-only answer anchored at the prompt after it grows beyond the space above the composer.

Change

Drive the recovery button from measured end position independently of follow intent. Keep the brief settling delay, but do not restart it on every streaming/layout report. Report end position after row and viewport size changes as well as scroll events. When a followed anchored turn outgrows the usable viewport, release its anchor and resume following the end; invalidate pending anchor positioning so it cannot pull the viewport back afterward. Manual navigation continues to opt out of follow.

This change applies to the shared web/desktop timeline and all providers rendered there. Native mobile scrolling is unchanged.

Scope and approval

Related to the maintainer-triaged missing-recovery-button bug #12372 and the viewport-position report #5903. Additional user report, synthetic evidence, and scope request.

This PR was opened before checking the existing report. The earlier obvious-bug-exception rationale is withdrawn. The existing issue's triage does not approve every behavior proposed here, especially releasing long-answer anchors. Maintainer review is requested on whether to narrow this PR to recovery-button reconciliation and handle following behavior separately. The synthetic checks do not establish the cause of all spontaneous upward jumps. No approval of this implementation is claimed.

Verification

• pnpm exec vp test run apps/web/src/components/chat/timelineEndAffordance.test.ts apps/web/src/components/chat/timelineScrollAnchoring.test.tsx apps/web/src/components/chat/MessagesTimeline.test.tsx apps/web/src/components/ChatView.logic.test.ts: 232 tests passed. New tests cover sustained off-end reports, transient settling, immediate hiding/reappearance, and cancellation. Existing geometry tests cover anchored-turn viewport overflow.
• pnpm --filter @t3tools/web typecheck: passed.
• Targeted vp lint: no errors; existing React compiler/dependency warnings in the surrounding ChatView and timeline components.
• Browser panel, isolated dev database, synthetic six-turn conversation: on the original code, set the viewport to the top without a manual wheel/key opt-out to simulate a stalled follow. Measured a 2,800px end gap and no return button. With this change, the same state exposes the button; clicking it reaches a 0px end gap and hides the button.
• Exercised anchor-ready/size-change callbacks against measured long-turn fixture geometry in the browser. The overflow path returns to the end and the delayed anchor-positioning callback does not pull it back. This is a synthetic layout check, not a live provider streaming run. Native mobile was not tested.

Before (off-end, recovery hidden):

After (same position, recovery available):

Recording: recovery button returns to the end

Implemented with GPT-6.1-Sol through the Codex harness in T3 Code.

[macroscopeapp]

Approvability

Verdict: Approved at 24c90c4

Macroscope's review found this PR approvable — This is a localized web timeline bug fix that restores the existing return-to-end affordance and prevents oversized streamed turns from remaining hidden beneath the composer. The new timing and geometry logic is small, isolated, and covered by targeted tests, with no schema, deployment, or sensitive-area impact.

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📥 Commits

Reviewing files that changed from the base of the PR and between 1e2ecbd and 24c90c4.

📒 Files selected for processing (4)

• apps/web/src/components/ChatView.tsx
• apps/web/src/components/chat/MessagesTimeline.tsx
• apps/web/src/components/chat/timelineEndAffordance.test.ts
• apps/web/src/components/chat/timelineEndAffordance.ts

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📝 Walkthrough

Walkthrough

The timeline now reports its end state after overflow measurement. ChatView uses those reports to control the delayed scroll-to-end affordance and adjusts anchor handling when content grows during live-follow.

Changes

Timeline end-state handling

Layer / File(s)	Summary
Timeline reporting and delayed affordance apps/web/src/components/chat/MessagesTimeline.tsx, apps/web/src/components/chat/timelineEndAffordance.ts, apps/web/src/components/chat/timelineEndAffordance.test.ts	MessagesTimeline reports a resolvable end state after scheduled overflow measurement, except during thread-position restoration, citation positioning, or when its data is not the current rows. The new controller hides the affordance when the timeline is at the end and schedules it to appear after 150 ms when the timeline is away from the end. Tests cover repeated reports, returning to the end, and cancellation.
ChatView integration and anchor handling apps/web/src/components/ChatView.tsx	ChatView routes end-state reports through the new controller. During live-follow, it scrolls to the end when the positioned anchor’s content overflows the usable viewport. After navigation, it retains guarded scroll-offset restoration. scrollToEnd clears the positioned and settled anchor refs.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant MessagesTimeline
  participant ChatView
  participant TimelineEndAffordance
  MessagesTimeline->>ChatView: Report resolvable end state
  ChatView->>TimelineEndAffordance: Report whether timeline is at end
  TimelineEndAffordance->>ChatView: Update affordance visibility immediately or after delay

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Suggested reviewers: maria-rcks

Merge Risk: ⚪ Minimal · up to 24c90

No actionable merge-blocking issue remains in the reviewed timeline changes; merge after normal checks.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the main change: restoring timeline follow behavior and bottom-navigation recovery.
Description check	✅ Passed	The description covers the required Problem, Change, Scope and approval, and Verification sections. It gives focused test results and states limitations. It also clearly says maintainer approval for t…

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[juliusmarminge]

Note

This comment is posted by Julius' dot

The recovery-button fix addresses #12372, but this also releases long-answer prompt anchors and resumes following, changing viewport behavior independently. The maintainer's triage of #12372 explicitly separates missing recovery from the stranded viewport in #5903, and the PR acknowledges that the anchor behavior lacks scope approval. Under one problem and prior approval, please narrow this PR to recovery-button reconciliation and discuss the anchor/following change separately with maintainers. Closing this combined submission; the reproduced recovery defect and its supplied evidence remain useful for a focused replacement.