fix(desktop): stop long-session transcript from drifting to old turns

[OutThisLife]

Symptom

In a long-running desktop session, the transcript would eventually drift up and show old responses while idle/waiting — purely visual, no functional impact (the session worked end-to-end).

Root cause

Regression from 9b8b054c2 ("off-screen turns skip rendering"), which added content-visibility: auto + contain-intrinsic-size: auto 37.5rem to every turn group in list.tsx.

contain-intrinsic-size: auto only remembers a turn's height after it renders. A turn that finished streaming near the bottom had its smaller mid-stream size remembered; when it later scrolled just off the top edge and content-visibility skipped it, it snapped back to that stale smaller height, shifting content down. Because the viewport uses overflow-anchor: none (so use-stick-to-bottom owns scrollTop), the browser can't self-correct — the bottom lock drifts and the view creeps up over older turns. The error is small per turn but accumulates over a long session, so it only shows up "eventually."

This is a well-known content-visibility failure mode

Not Hermes-specific — it's the documented CLS/scroll-jump behavior of content-visibility + contain-intrinsic-size when the placeholder estimate differs from real height and the shift lands above the scroll position:

• MDN and the browser-rendering guide describe it directly.
• Tencent's Hy3-preview chat removed content-visibility entirely for the identical reason ("scroll-anchoring does not reliably compensate for size changes triggered by content-visibility transitions"), and recommended, if re-adding: apply it only to older rows, with per-row measured intrinsic sizes rather than a fixed guess. This PR is the "only older rows" half of that.

Fix

Exempt the newest turns (a small live tail) from virtualization, so a turn is only ever skipped after its layout has settled at its final size (remembered == real → skipping it changes no height). Off-screen older turns still skip, so the dialog/popover whole-document recalc win on long transcripts is fully preserved (it scales with the hundreds of old turns, not the small tail).

Extracted a pure, tested helper (isVirtualizedGroup + LIVE_TAIL_GROUPS = 6) rather than inlining the boundary.

History worth knowing (a latent mismatch this sidesteps)

overflow-anchor: none on the viewport is a leftover from the virtualizer era. The transcript used to be a TanStack-Virtual component (thread-virtualizer.tsx, now deleted); overflow-anchor: none was added specifically for it in e67ab2e04 / #37866 ("stop chat scroll jumping by disabling native scroll anchoring") so only the virtualizer compensates. Then 76b93869d ("rebuild thread autoscroll on use-stick-to-bottom") replaced the virtualizer with the current content-visibility + use-stick-to-bottom list — but left overflow-anchor: none behind.

So the transcript now pairs content-visibility (which relies on scroll anchoring to smooth placeholder↔real deltas) with overflow-anchor: none (which disables it) — which is why we're more exposed to this than a typical app.

Deeper alternatives (not done here; this PR is the minimal safe fix)

1. Native scroll anchoring: drop overflow-anchor: none, set overflow-anchor: auto on the viewport + none on children + a 1px bottom sentinel, and let the browser compensate for above-viewport size deltas. Precedent: svelte-virtual-chat #55, aura-os.
2. Back to an end-anchored virtualizer: TanStack Virtual now has first-class chat support (#1173: anchorTo: 'end', followOnAppend, scrollToEnd) — and the sidebar already uses @tanstack/react-virtual.

Verification

• New unit tests for isVirtualizedGroup (live tail always rendered, older turns virtualized, short-transcript, custom tail) — list.test.ts 12/12.
• Full thread/ suite: 56/56.
• tsc typecheck clean; eslint clean; prettier clean.

Note: jsdom has no real layout / content-visibility, so the scroll-drift itself can't be reproduced in a unit test — the helper boundary is tested; the visual fix needs a real long-session spin to confirm.

[github-actions]

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_{running on 94f6dff}

CI timings

CI timings · View job

Wall time 7m30s vs 7m42s (-2.6%). 4 job(s) slower, 1 faster, 2 unchanged.

• OSV scan / Scan lockfiles / osv-scan: +25.0s
• Build&Test Docker image / build (arm64, ubuntu-24.04-arm, linux/arm64, type=gha,scope=docker-arm64, type=gha,mode=max,scope...: -15.0s
• Build&Test Docker image / build (amd64, ubuntu-latest, linux/amd64, type=gha,scope=docker-amd64, type=gha,mode=max,scope=do...: +6.0s
• Detect affected areas: +4.0s
• OSV scan / Emit review status: +3.0s

[alt-glitch]

This was generated by AI during triage.

Related to #40433 and #43914 in the desktop scroll-stability family. This patch addresses a distinct content-visibility/intrinsic-size mechanism in the assistant-ui thread list.