The desktop chat GUI pinned the viewport to the bottom on every content
growth while a turn streamed, so the window chased tokens as they arrived.
Remove that follow behavior: once a turn is running the viewport stays
exactly where the user left it.
- Delete the streaming ResizeObserver re-pin loop in useThreadScrollAnchor.
- Delete the post-run bottom lock (kept pinning ~1.2s after completion).
- Keep the one-time jump-to-bottom on user submit / new turn / session
change so a freshly submitted message still lands in view.
- Update streaming.test.tsx to assert the viewport no longer follows
streaming growth or snaps down on final code-highlight remeasure.
e029b7597b