[Bug]: Zooming in can break streaming chat scroll-follow on macOS #25692

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

T3 Code desktop app on macOS (apps/web), chat timeline. I use View → Zoom In / ⌘+.

Steps to reproduce

1. Open a long thread, start a response, and stay at the bottom while it streams.
2. Choose View → Actual Size / ⌘0, then Zoom In / ⌘+ four times (144%). Six steps is about 172.8%.
3. Click empty space in the timeline header or padding, away from the scrollbar, and let the response continue.
4. Repeat with the window at the supported 840×620 minimum size.

I see this across threads. It can happen while I stay in T3; switching apps or monitors makes it more noticeable. I have not confirmed that physically moving the window between displays causes it.

Expected behavior

When I am following the newest output, zooming or clicking empty space in the timeline should keep it visible. If I scroll up, drag the scrollbar, or use the keyboard to read older messages, following should release.

Actual behavior

The timeline stops following, new output lands below the window, and Scroll to end appears. Clicking it gets me back to the bottom briefly, but the problem can happen again. I also see it during ordinary zooming or streaming without intentionally scrolling up.

What I found

My testing points to two problems: clicking empty timeline padding can be mistaken for scrollbar input, and zoom or viewport changes can be mistaken for me scrolling upward. The current PR fixes one zoom case; my additional patch covers these other cases.

At 840×620 and 144%, I reproduced a failure on the exact PR head: it ended 1,281.36 CSS pixels above the bottom. With my additional patch, the same replay ended within 0.14 CSS pixels. That is one test result, not a guarantee for every machine or zoom setting.

Impact

Major degradation or frequent failure

I repeatedly have to return to the bottom during long runs.

Version or commit

I first saw this on 0.1.3-nightly.20261026.3740. I reproduced it again on 0.1.4-nightly.20261027.3783.

Environment

macOS 27.0.1, arm64. I am using app zoom, not text-size or display scaling. The full-app replay used the Nightly 3783 client and an isolated copy of its backend with synthetic streaming. It was not a live provider run.

Screenshots and supporting files

The patch and test results are in my comment on PR #25738.

Before — the PR as it stands:

After — with my additional patch:

Workaround

The Scroll to end button only helps briefly. My local v6 workaround is working in normal use, but it is not an upstream fix. PR #25738 is still open; the author needs to apply and verify the additional patch.