fix(web): scrolling over an inline HTML render no longer snaps back to the bottom

[josephv123]

Problem

Scrolling up with the pointer over an agent's inline HTML render didn't stop live-follow, so the next layout change snapped the thread back to the bottom (#17056). The frame is a sandboxed, opaque-origin iframe. Wheel and touch input over it go to the frame's own document, and none of the timeline's opt-out listeners (wheel, touchmove, pointerdown, keydown) fire. The browser still chains the scroll out to the timeline, so the view moved while liveFollowEnabled stayed true, and maintainScrollAtEnd re-pinned the end on the next resize, image load, or frame height report.

Change

The opt-out listeners in ChatView gain a scroll listener. While the timeline is following, a scroll counts as the reader leaving follow mode when all three hold:

• scrollTop went up by more than 1 px in total since the last move toward the end. Slow scrolls that move under a pixel per event add up.
• The content didn't get shorter. Shrinking content, such as a collapsed disclosure, can only pull the offset up by clamping, so it is excluded.
• The viewport is now outside the follow re-arm band (viewportIsAwayFromEnd, the same check the touch and pointer paths use).

Following itself only moves the timeline toward the end, so this doesn't fire while pinned. Streaming growth doesn't lower scrollTop, so the #5566 flicker guard still holds.

The upward-move check is a small pure helper, createUpwardScrollDetector in timelineScrollTarget.ts, with focused tests. This is option 1 from the maintainer's triage. It covers HTML renders, MCP app frames, and renders published before #16283, since it doesn't depend on the frame's bootstrap.

Scope and approval

Fixes #17056, triaged and confirmed by @juliusmarminge, who recommended this host-side approach as "the smaller, more general fix": #17056 (comment)

Verification

All checks below used a real mouse wheel (Playwright with chrome-headless-shell 154) against a dev server seeded with a thread that ends in a tall KaTeX html_render page. Each run scrolled two wheel ticks up, then resized the window by 20 px.

Pointer position	main (04ad17425a)	This branch
Over the html_render iframe (the timeline sees 0 of 2 wheels)	300 px up, then 0 px after the resize (snapped back)	300 px up, then 346 px after the resize (stays)
Over the timeline gutter (control, the timeline sees 2 of 2)	stays (346 px)	stays (346 px)

Before (main) and after (this branch). The thread and page are a neutral fixture. The red dot is the real pointer, and the bottom-right counter is the distance from the end. In each video, the first half scrolls over the HTML render and the second half scrolls over chat text as a control.

Before, on main: the scroll over the render snaps back to the bottom after the layout change. The control stays.

After, on this branch: both stay where they were scrolled.

MP4 versions: before, after.

Regression check for live-follow. I opened the thread, resized the window, then sent a turn that streamed 80 lines, about 2,100 px of growth, sampling every 250 ms. On both main and this branch the thread stayed pinned: 0 px from the end after the open and after the stream, and the scroll-to-end pill never appeared. The largest gap mid-stream varied between runs (241–455 px on this branch, 267 px on main) because of sampling against the follow scroll. A temporary log on the new opt-out path never fired during the open or the stream.

• vp test run src/components/chat/timelineScrollTarget.test.ts: 18 passed, including 4 new tests for the detector.
• tsc --noEmit for apps/web passes. Lint on the changed files adds no warnings (86 before and after, all existing memo-dependency warnings elsewhere in ChatView.tsx).

Not checked: touch scrolling on mobile web, and MCP app frames (I had no MCP app fixture). The native mobile app uses its own timeline and is not affected by this change.

Claude Opus 5.5 via Claude Code in T3 Code

[macroscopeapp]

Approvability

Verdict: Approved at d60783e

Macroscope's review found this PR approvable — This is a focused, tested web bug fix that adds guarded scroll detection for embedded-frame navigation and preserves existing scroll handlers and cleanup. Its runtime impact is narrow, with no schema, infrastructure, security, billing, or default-setting changes.

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📒 Files selected for processing (2)

• apps/web/src/components/chat/timelineScrollTarget.test.ts
• apps/web/src/components/chat/timelineScrollTarget.ts

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• apps/web/src/components/chat/timelineScrollTarget.ts
• apps/web/src/components/chat/timelineScrollTarget.test.ts

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📝 Walkthrough

Walkthrough

The timeline now detects upward scrolling, including scrolls chained from embedded frames. When live-follow is active and the viewport moves away from the end, the existing manual-navigation handler runs.

Changes

Timeline live-follow

Layer / File(s)	Summary
Upward-scroll detection apps/web/src/components/chat/timelineScrollTarget.ts, apps/web/src/components/chat/timelineScrollTarget.test.ts	createUpwardScrollDetector detects upward movement from successive scroll measurements. It ignores movement toward the end and content growth, resets its baseline after content shrinkage, and supports cumulative subpixel movement. Tests cover these cases.
Live-follow integration apps/web/src/components/ChatView.tsx	A scroll listener uses the detector and invokes the existing manual-navigation handler when live-follow is active and the viewport is away from the end. Cleanup removes the listener.

Priority: ⬇️ Low

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

Change: Bug fix · Severity of issue fixed: Low

Merge Risk: ⚪ Minimal · up to b2d72

No actionable issue was established for this change; it is mergeable after normal checks.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy the coding requirements in [#17056]. ChatView adds a passive scroll listener and removes it during cleanup. createUpwardScrollDetector detects cumulative upward movement, ign…
Out of Scope Changes check	✅ Passed	The production changes and tests stay within [#17056]. They add the host-side scroll detection required for embedded render scrolling and test that detector. No unrelated behavior or unrelated test ar…
Description check	✅ Passed	The description includes all required sections: Problem, Change, Scope and approval, and Verification. It explains the bug, implementation, linked issue and maintainer approval, focused test results, …
Title check	✅ Passed	The title is concise, specific, and accurately describes the primary fix for inline HTML render scrolling and live-follow snapping.

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Inline comments:
Review comments at @apps/web/src/components/chat/timelineScrollTarget.ts:
- Around line 43-44: Update the upward-scroll detection around `movedUp` so
successive upward movements of at most 1 px accumulate against a retained
baseline instead of resetting it on every event. Reset the baseline when content
shrinks or scrolling changes direction, and preserve the existing detection
behavior for larger upward movements.

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📒 Files selected for processing (3)

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

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