Desktop: chat viewport snaps back to bottom while scrolling up during streaming response #40322

Environment

OpenCode Desktop on Windows 11 (also affects the web UI).

Description

While an assistant response is streaming, scrolling up (mouse wheel or PageUp/PageDown) is immediately undone: the viewport is pulled back to the bottom on the next streamed token. The behavior persists until the app is restarted.

Related: #29094 (TUI), #17996, #20600.

Steps to reproduce

1. Send a long prompt that streams a response.
2. While tokens are still arriving, scroll up with the mouse wheel or PageUp.
3. The viewport snaps back to the bottom within a second.
4. Repeat a few times - the behavior never recovers until restart.

Expected behavior

Scrolling up during streaming keeps the viewport in place; scrolling back to the bottom resumes auto-follow.