[dovvnloading]

Problem

ChatPage.renderCost.test.tsx > "follows streamed output instantly, and animates only the jump the reader asks for" fails intermittently with AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times:

• Locally, it failed in 3 of 4 full runs on a loaded machine (2026-10-17).
• In CI, it failed the "Frontend checks" job of test(execution): start the job-object tests on the real interpreter #325, a Python-only change: 1 failed, 1046 passed.

It blocks merging unrelated pull requests.

Root cause

The streaming bubble shows text through useThrottledText, which delivers each change from a setTimeout. The test:

1. called stream("more ");
2. cleared the mock;
3. clicked "Jump to latest" as soon as the button existed.

The button already exists after the earlier "tok " delivery, so the "more " delivery could still be pending at the click. When that delivery's render landed in the same act as the click, it ran after jumpToLatest had set the reader back at the end. followStreamedOutput then, correctly, followed the new text with an instant scroll, and the test counted two scrolls.

The product behaves as designed: a reader who jumped to the end is followed when new text arrives. Only the test's timing assumption was wrong.

Change

Test-only. Each step waits until its text is in the bubble before going on, so nothing is pending at the click. The mock is cleared right after scrolling away, and a new assertion checks that delivering "more " while scrolled away scrolls nothing. The existing assertions are unchanged:

• every follow is { behavior: "instant" };
• the click causes exactly one { behavior: "smooth" } scroll.

Checks

• npx vitest run src/features/chat/ChatPage.renderCost.test.tsx, 10 consecutive runs: 10 passed, 0 failed.
• npx eslint on the file: clean. npx tsc --noEmit -p .: clean.
• Pre-push hook (scripts/check.ps1 quick tier): all checks passed. Backend 3071 passed; vitest 77 files, 1047 tests passed.

Limits

• Root cause is from reading, not a reproduction. The intermittent failure was not reproduced on demand on a quiet machine. The cause follows from the throttle's timer and the order of work inside the click's act.
• A product observation, not changed here. While a reply streams, the next delivery after a jump follows with an instant scroll. That can cut the smooth jump animation short. It is a cosmetic matter of animation, not a defect in following.

Plan item: none (flaky test)

🤖 Generated with Claude Code