TUI: option to keep the scroll position when a prompt is submitted #53186

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Submitting a prompt in a session always scrolls the transcript to the bottom (toBottom() from the session prompt's onSubmit). If you had scrolled up to read earlier output, that position is lost the moment anything is submitted.

This is most disruptive when prompts are not typed by the person reading: tools that drive the TUI (multiplexers, orchestrators, scripts typing into the composer) submit follow-up prompts while you are reading further up, and every one of them yanks the view back down.

Proposal: an opt-in TUI setting, keep_scroll_on_submit in tui.json (default false, so current behavior is unchanged), plus a runtime toggle (/keep-scroll in the command palette, persisted like /timestamps). When on, submitting leaves the viewport where it is. A viewport already at the bottom keeps following new output via the existing sticky scroll, so nothing changes for users who are not scrolled up.

Steps to see the current behavior:

1. Get a session transcript taller than the terminal.
2. Scroll up a few pages (PageUp).
3. Submit any prompt.
4. The view jumps to the bottom.

OpenCode version: dev @ 907b3bc