tui: ctrl+b background hint shown but keypress does nothing

Description

The v2 TUI shows a ctrl+b background hint under a running foreground subagent task, but pressing ctrl+b does nothing: the subagent keeps blocking the parent session, nothing is detached, and there is no error or toast. The hint appears to be intentional (there is a session_background keybind, a hidden session.background command, and a POST /experimental/session/:sessionID/background endpoint), so the keypress itself seems broken rather than the hint being stale.

Related but distinct: #36940 (hint renders when child work is already backgrounded).

Plugins

None

OpenCode version

2.0.3

Steps to reproduce

1. Run with experimental background subagents enabled (the hint only renders when the capability is on).
2. Prompt the main session to spawn a foreground (synchronous) subagent via the task tool (no background: true).
3. While it runs (task row shows e.g. ↳ Bash Running …), observe the hint under it: ctrl+g view subagents · ctrl+b background.
4. Press ctrl+b — both in the main transcript and while inside the subagent's session view (ctrl+g).
5. Nothing happens.

Screenshot and/or share link

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Operating System

macOS (Darwin 27.0.0, arm64)

Terminal

Ghostty (TERM=xterm-ghostty, COLORTERM=truecolor)