[Bug]: Minimap down button stays active at the bottom of a thread #33027

Before submitting

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

Area

apps/web

Steps to reproduce

1. Open a thread whose last two or more turns fit on one screen.
2. Scroll all the way to the bottom.
3. Hover over the left edge of the chat to show the minimap and look at its down button.

Expected behavior

At the very bottom of the thread, the down button is disabled and clearly muted, dimmer than it is today, so it is obvious there is nowhere further to go. The up button behaves the same way at the very top.

Actual behavior

The down button looks the same as when there is more to scroll. The buttons only get the disabled style (64% opacity) when the current turn is the first or last turn, and the current turn is the first user message in view (resolveTimelineMinimapCurrentIndex). At the bottom of a thread whose last few turns fit on one screen, that is not the last turn, so the down button stays enabled. Basing the state on the scroll position (at the top / at the bottom) would cover this case.

Found by reading TimelineMinimap.tsx while looking into #33026; there is no recording for this one.

Impact

Cosmetic issue

Version or commit

Desktop 0.1.7 (AppImage). The minimap code involved is unchanged on main @ 6577c6b.

Environment

NixOS 26.11, Wayland, 3840×2160 display