feat(web): show messages below in the scroll-to-end button

[saphid]

What Changed

When reading earlier messages on web or desktop, the scroll-to-end button now shows how many messages remain below the visible area, such as “5 messages.” Clicking still returns to the end. The accessible label includes the count and action.

Why

“Scroll to end” gives no indication of how much conversation remains. This follows the interaction in the reference video.

The count includes a partially visible message until its bottom clears the composer. Tool rows and messages hidden inside collapsed turns are excluded; if only non-message content remains, the button retains “Scroll to end.” The calculation uses cached virtual-list positions, the measured header, and the actual composer height, including when the composer collapses. Each scroll lookup is logarithmic.

UI Changes

The review fix in 6e709c8295 adds finite-measurement guards and eight regression cases. The media below remains representative: finite row geometry and rendered UI are unchanged.

Captured on lxso2 in Chromium at 1280 × 900, dark theme, using the same synthetic conversation of 24 messages across 12 completed turns. Base: 3e6f856f23; candidate: 464fb955b0 (the captured overlay was verified byte-for-byte against the committed patch).

Before: the label stays “Scroll to end” as the conversation scrolls.

Button detail at the first scrolled position:

After: the same scroll positions show 5, 8, and 2 messages. Clicking returns to the end and hides the button.

Button detail at the first scrolled position:

The primary GIFs retain the entire chat pane, including the conversation and complete composer; only the unrelated sidebar is removed. They are sampled at 15 fps with unchanged playback speed and show the initial scroll through the return to the end. Button details are supplemental stills from the same first scrolled state. Before full-window video · After full-window video.

Before/after screenshots and additional states

Before

After

Singular label

Narrow web layout, light theme

Verification

All final checks ran on lxso2:

• vp test run apps/web/src/components/chat/messagesBelow.test.ts apps/web/src/components/chat/MessagesTimeline.logic.test.ts apps/web/src/components/chat/MessagesTimeline.test.tsx: 163 passed.
• vp run --filter @t3tools/web typecheck: passed.
• Targeted lint on the four changed TypeScript files: 0 errors; 29 existing warnings.
• Browser assertions: counts match the visible message boundary and fixture order at three scroll positions, including unmounted rows; scrolling changes the count both ways; clicking returns to the end and hides the button. Singular label, viewport resize, 390-pixel light layout, and reduced-motion return-to-end passed. No page errors in the comparison runs.
• git diff --check: passed.

Web and desktop share this renderer. Electron shell behavior was not exercised separately. No native mobile, provider, or wire-contract changes; the calculation is client-local for local and remote connections.

Independent review was attempted directly on lxso2 with claude --bare -p --model claude-opus-5 --effort high --tools '' --no-session-persistence --output-format json. Exit status 1: OAuth session expired and could not be refreshed. No reviewer model ran. The same authentication failure also occurred in the earlier Mac attempt.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Implemented with GPT-6 in the Codex harness.

Summary by CodeRabbit

• New Features

  • The scroll-to-end button now shows how many messages remain below the current view.
  • The count updates as you scroll, messages arrive, content streams, or the composer and viewport resize.
  • Improved handling of partially visible messages and overlay space.
  • The button’s accessible label now reflects whether it will return to the latest message or show a specific number of messages.

• Documentation

  • Added guidance for returning to the latest message on web and desktop.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — The PR adds a new production interaction that calculates and displays messages below the viewport, integrating virtualized timeline measurements with scrolling, resizing, streaming growth, and composer overlays. Its runtime behavior spans the timeline and existing scroll-to-end control rather than being limited to tests or documentation.

^{You can add or adjust custom eligibility rules. Learn more.}

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📥 Commits

Reviewing files that changed from the base of the PR and between 464fb95 and 6e709c8.

📒 Files selected for processing (2)

• apps/web/src/components/chat/MessagesTimeline.logic.ts
• apps/web/src/components/chat/messagesBelow.test.ts

🚧 Files skipped from review as they are similar to previous changes (2)

• apps/web/src/components/chat/messagesBelow.test.ts
• apps/web/src/components/chat/MessagesTimeline.logic.ts

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📝 Walkthrough

Walkthrough

The chat timeline now counts message rows below the unobscured viewport. ChatView displays that count in the scroll-to-end pill, updates its accessibility label, and passes the composer overlay height to the timeline. Tests and composer documentation cover the behavior.

Changes

Message count below viewport

Layer / File(s)	Summary
Counting logic and validation apps/web/src/components/chat/MessagesTimeline.logic.ts, apps/web/src/components/chat/messagesBelow.test.ts	countTimelineMessagesBelow counts indexed message rows below the visible boundary. Tests cover scrolling, resizing, overlays, virtualization, invalid measurements, and logarithmic lookup behavior.
Timeline reporting and metrics apps/web/src/components/chat/MessagesTimeline.tsx	MessagesTimeline reports message counts through onMessagesBelowChange. It combines the visible bottom inset with the header size and updates counts from layout, overflow, and LegendList metric changes.
Scroll-to-end display and documentation apps/web/src/components/ChatView.tsx, docs/user/composer.md	ChatView renders the message count in the scroll-to-end pill and its accessibility label. The button uses a data attribute for overlay clearance lookup. The composer documentation describes the behavior.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Severity of issue fixed: Low

Merge Risk: ⚪ Minimal · up to 6e709

The scroll-to-end control now shows the number of messages below the visible conversation area while preserving its existing action. No current merge-blocking risk is identified.

Sequence Diagram(s)

sequenceDiagram
  participant User
  participant ChatView
  participant MessagesTimeline
  participant LegendList
  ChatView->>MessagesTimeline: Pass composer overlay height
  LegendList->>MessagesTimeline: Report scroll and header metrics
  MessagesTimeline->>MessagesTimeline: Count rows below visible viewport
  MessagesTimeline-->>ChatView: Send message count
  ChatView-->>User: Render count and scroll-to-end control

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 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.
Description check	✅ Passed	The description includes all required sections: What Changed, Why, UI Changes, and Checklist. It explains the behavior, provides before/after screenshots and videos, and documents verification results…
Title check	✅ Passed	The title clearly and concisely describes the primary change: showing the number of messages below the viewport in the scroll-to-end button.

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Actionable comments posted: 1

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Inline comments:
In `@apps/web/src/components/chat/MessagesTimeline.logic.ts`:
- Line 195: Update the row-position calculation around positionAtIndex so only
finite height values are treated as measured; for undefined, NaN, or infinite
heights, fall back to a finite next-row position, preserving the existing top
fallback when necessary so bottom remains usable for partial-message detection.

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: 091bb82c-6d6e-498f-865f-2319f2dc30fd

📥 Commits

Reviewing files that changed from the base of the PR and between 6c58362 and 464fb95.

📒 Files selected for processing (5)

• apps/web/src/components/ChatView.tsx
• apps/web/src/components/chat/MessagesTimeline.logic.ts
• apps/web/src/components/chat/MessagesTimeline.tsx
• apps/web/src/components/chat/messagesBelow.test.ts
• docs/user/composer.md

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