Narrow side-chat composer collapses when opening the model picker

[celeroncoder]

Summary

When the side-chat panel is narrow, the composer starts as a pill. Clicking the pill expands it, but clicking the model selector immediately collapses the composer again instead of opening the model picker.

The model picker works normally when the panel is wide enough for the full composer to stay visible.

Versions and environment

• bb 0.39.0 (web app), Ubuntu 24.04
• pi, Gemini 3.7 Flash via CLIProxyAPI
• local environment, bb Connect paired

Steps to reproduce

1. Open a side chat in a narrow panel.
2. Click the composer pill to expand it.
3. Click the model selector.
4. The composer collapses back into a pill.
5. The model picker never opens.

Expected vs actual

Expected: The model picker should open and the composer should stay expanded.
Actual: The composer collapses back into the pill, so the model picker can't be used.

Evidence

Screen.Recording.2026-08-25.at.12.59.58.PM.mov

What you ruled out

• The model picker is not generally broken. It works when the side-chat panel is wide.
• This only reproduces at narrow side-chat widths.
• No matching open BB issue or PR was found.

Suggested priority and effort (optional)

Medium - The side chat still works if the panel is widened, but model switching is blocked at narrow widths. Suggested effort is small to medium, depending on where the responsive collapse state is handled.

Checks

• I reproduced this on the latest release or on main, or I say above that I could not.
• I searched open and closed issues for the same problem.
• If an agent wrote this, the body ends with > AGENT GENERATED and links the thread or report.

AGENT GENERATED