[Bug]: ACP (Copilot) thread stays on "Thinking" after the final reply; queued and child-thread messages never deliver #36108

Before submitting

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

Area

Desktop app, ACP provider (acpRegistry: GitHub Copilot)

Steps to reproduce

1. Start a thread on the Copilot ACP provider (model Claude Opus 5.5, reasoning effort high, full-access).
2. Send any message. The agent runs its tool calls and writes its final reply.
3. Optional, makes it worse: the parent thread launches a child thread with t3_thread_launch, and the child reports back with t3_thread_send (mode auto).

Expected behavior

The turn ends once the final reply has been written. The composer goes back to idle, and any message sent to the thread (by the user or by a child via t3_thread_send) starts the next turn.

Actual behavior

After the final reply has fully rendered, the thread stays on "Thinking" with the Stop button showing, indefinitely. It happens on most turns, with or without a child thread.

• Messages the user types are queued (dashed bubble with edit / x) and never start.
• Messages from the child thread via t3_thread_send are also queued ("Send now" / edit / x) and never reach the agent.
• The only way out is pressing Stop ("Run interrupted · Stopped by user"). Child messages queued before that stay in the queue unread, so the agent never sees them unless the user clicks "Send now".

Impact

Every turn needs a manual Stop. Parent/child loops (the child reports, the parent fixes, the child retests) break silently: the child's reports are lost in the queue while the parent looks busy.

Version or commit

T3 Code server 0.1.9

Environment

macOS (arm64), desktop app; provider instance acpRegistry_copilot

Logs or stack traces

Not captured yet; happy to collect them if you can point me to the right log file.

Screenshots, recordings, or supporting files

The final reply is complete, but the status still shows "Thinking" and the user's next message is queued.

Workaround

Press Stop after each reply. Then click "Send now" on each queued message, or ask the agent to read the child thread with t3_thread_read.