test(desktop): settle prompt-rail scroll jumps

[liugddx]

Summary

Make prompt-rail test scrolling deterministic for the turn virtualizer.

• explicitly dispatches the transcript scroll notification after assigning scrollTop;
• waits two painted frames before the caller asserts the mounted turn window;
• keeps the exact tail-mount, eviction, focus-return, and selection-collapse assertions unchanged.

Why

scrollTranscriptTo previously only assigned scrollTop. A programmatic assignment did not provide a deterministic observation/paint boundary for the virtualizer, so the eviction scenario could wait ten seconds for turn-prompt-rail-120 while the mounted window remained stale.

This reproduced on an unrelated #3789 head in run 32930874507. That PR has no Desktop/UI diff.

The spec already used notifyTranscriptScrolled and waitForPaintedFrames later in the same eviction journey. This change moves that existing boundary into the shared scroll helper so every top/bottom setup has deterministic semantics.

Verification

• npm run lint
• npm run format:check
• npx knip --workspace apps/desktop
• git diff --check

The local Windows full build currently reproduces unrelated current-main Runtime Host type failures, while the exact main push CI is green. The authoritative Desktop e2e evidence for this test-only change is therefore the hosted Linux/xvfb test check on this PR.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool and scope: Codex analyzed the hosted trace, reused the spec's existing scroll notification and paint helpers, and ran the listed local gates.

Fixes#3862.

Checklist

• Existing assertions cover the failed boundary without being weakened
• Lint, format, Desktop knip, and diff checks pass locally
• Hosted test passes on exact head 52e9a6024, including the full Desktop e2e suite

Does this PR entail a change in behavior?

• Yes
• No - this changes only Desktop e2e synchronization.

[liugddx]

@Astro-Han Hosted exact-head est is green, including the full Desktop e2e suite. The change only reuses the spec's existing scroll notification and painted-frame helpers; all exact tail-mount, eviction, focus, and selection assertions remain unchanged. Please review when available.

[Astro-Han]

I reviewed this head and found no blocking issues.

Adds notifyTranscriptScrolled after scrollTranscriptTo to ensure virtualizer observes programmatic scroll; hosted test: SUCCESS (32932185830).

No P0-P3.

简体中文

该头无阻断。

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Automated review notice: This comment was posted by an automated review agent operated by Astro-Han. It is not an independent human review and does not replace one.