fix(console-log): stop dragging the reader back to the newest line - #3872

Closes #3838.

The console log scrolled to the bottom on every append:

useEffect(() => {
  if (!logRef.current) return;
  logRef.current.scrollTop = logRef.current.scrollHeight;
}, [logs]);

So scrolling up to read an older line lasted until the next entry arrived. And something is always appending — a combo fanning out to several models, a background token refresh — which is what makes it unreadable rather than merely awkward. The reporter could not see which member of a fusion had failed.

The change

Following the tail becomes a property of where the reader already is: at the bottom means keep me there, anywhere else means leave me alone.

Two details that are not obvious:

• The position is recorded on scroll, not read inside the effect. By the time the effect runs, the new lines are already laid out, so the element no longer reports where the reader was before they arrived.
• The comparison carries a few pixels of slack rather than testing equality. scrollHeight and clientHeight are integers while scrollTop is fractional under a non-integer device pixel ratio, so an element sitting at its bottom routinely reports a residue of a pixel or two. An exact test drops the reader out of follow mode without them having scrolled.

The decision lives in a plain function (src/shared/utils/scrollFollow.js) because tests/vitest.config.js runs with environment: "node" and there is no React testing setup — a component test could not have run at all, and I would rather have a real test of the rule than none.

Verification

tests/unit/scroll-follow.test.js — 7 passed. Each claim checked by reverting it:

npx next build — ✓ compiled successfully.

Not changed

EndpointPageClient.js has the same unconditional scroll for the Tailscale install log. Left alone deliberately: it runs for the length of one install and has nothing older to read. The helper is there if you want it applied.