Messages submitted while background subagents are running get stranded in the Queued (N) UI region and only drain via bg-agent completion (workaround: 'submit another message' is a placebo)

## Summary

Messages submitted while the main agent is in the "Waiting for background agents" / deferred-idle state get stranded in the separate `Queued (N)` UI region and do not automatically drain at the next available opportunity. The only way to get a stranded message processed (without destroying it via Esc/double-Esc) is to **submit another message** — which by itself doesn't drain the queue either, but it advances the user-visible state in a way that often coincides with a background subagent completion, making it appear as though the second submission "pushed the first one through."

(This investigation was performed collaboratively with Copilot CLI itself. I used the CLI in this session to both reproduce the symptom and dig through the minified `app.js` shipped in `@github/copilot@1.0.48` to identify the precise control flow that causes it. The technical analysis below is the agent's findings, reviewed by me.)

## Version

`GitHub Copilot CLI 1.0.49-1` (also reproduced symptom in 1.0.48 via source inspection).

## Observed behavior

The CLI has two distinct queueing UIs for input submitted while the agent is busy:

1. **Inline pending** (normal case): when the agent is actively executing a tool or streaming a response, a message submitted with Enter appears under the active turn in the main conversation window with a `└ [pending]` decoration. It is consumed at the next turn boundary.

2. **Separate `Queued (N)` region** (the bug surface): when the agent's turn ended while a background subagent (`task` tool with `mode: "background"`, etc.) is still running, a message submitted with Enter lands under a separate `Queued (N)` header rendered below the input prompt — visually similar to but distinct from the inline pending UI.

Once in the `Queued (N)` region, the message can stay there indefinitely. Subsequent user input does not reliably drain it. In one of my real long-running sessions, the same status-update request had to be re-typed multiple times across the session before getting through (resulting in turns 17, 14, 8, 7, 6, 5, 4, 2, 1 having NULL assistant responses in the session store — they were submitted but never processed).

## Root cause (from minified source inspection of `app.js`)

The relevant control flow in the `processQueuedItems` path:

```js
enqueueItem(e) {
this.addItemToQueue(e);
this.emitEphemeral("pending_messages.modified", {});
if (!this.isProcessing) {
// 🔴 The bug guard:
if (e.kind !== "resume_pending" && this.hasActiveBackgroundWork()) return;
this.processQueue().catch(...);
}
}

hasActiveBackgroundWork() {
return this.taskRegistry
.list({ includeCompleted: false })
.some(e => e.type === "agent" && e.status === "running");
}

// At the end of a turn:
if (this.hasActiveBackgroundWork()) {
this.idleDeferredByBackgroundWork = true;
this.idleDeferredAborted = e;
} else {
this.emitSessionIdle(e);
}

// In the bg-task completion callback:
this.taskRegistry.setOnCompletionCallback(s => {
// ... record completion, emit notifications ...
this.emitDeferredSessionIdleIfReady();
});

emitDeferredSessionIdleIfReady() {
if (!this.idleDeferredByBackgroundWork || this.isProcessing || this.hasActiveBackgroundWork()) return;
if (this.itemQueue.length > 0) {
this.processQueue().catch(...); // ← the drain path
return;
}
this.emitSessionIdle(this.idleDeferredAborted);
}
```

The trap: `enqueueItem` adds messages to `itemQueue` but **refuses to start `processQueue()` whenever any background agent is running**. The only mechanism that drains `itemQueue` in this state is `emitDeferredSessionIdleIfReady`, which fires from a `taskRegistry` completion callback. So the queue can only drain when **all** running subagents complete AND the bg-task callback for the most-recently-completed one fires.

Two ways this becomes a user-visible stall:

* If new background subagents are continuously dispatched (autopilot, multi-step planning), there is never a clean window with `hasActiveBackgroundWork() === false`, so the drain path is never taken.
* The completion callback path can race with re-entering active processing (e.g., the main agent has resumed work for some other reason), and the queue is left waiting for the next cycle.

## Why "submit another message" sometimes appears to work as a workaround

It doesn't actually drain the queue directly — the `hasActiveBackgroundWork()` guard rejects the new submission for the same reason. But submitting another message:

1. Pushes a new entry into `itemQueue`, which forces a `pending_messages.modified` ephemeral event and re-renders the queue UI.
2. Often happens to coincide with a background agent's completion callback firing (because the user typically only retries when they perceive a delay, which is itself correlated with bg agents finishing up).

So from the user's perspective, "I submitted a second message and the first one went through" — which gave me the wrong initial mental model. The actual cause is the bg agent completing, not the resubmission.

## Reproduction

Empirically confirmed in this conversation by intentionally triggering "background agent in flight + main agent actively running a sync tool call" simultaneously. Submitting a message during the overlap window landed it in the separate `Queued (N)` region. The message remained there until both:

1. The 90-second background `Start-Sleep` agent completed, AND
2. The 60-second main-agent sync tool call completed.

Once both conditions cleared, the queued message drained on its own and was processed normally.

We also tried isolating each condition (bg-only, main-busy-only) and neither alone reproduced — both must be active simultaneously, and we could only reliably trigger it when the main agent was doing meaningful in-flight work (not just idle waiting on bg agents).

Repro recipe in code form, runnable inside `copilot`:

```
Do exactly two things in this single turn:
(1) Dispatch a background-mode task subagent that sleeps for 90 seconds.
(2) Immediately after, run a SYNC powershell tool call `Start-Sleep -Seconds 60` with initial_wait 60.
Do not end your turn until that sync command has completed.
```

While the agent is in step 2, submit any test message via Enter. It lands in `Queued (N)`.

## Expected behavior

Queued user messages should drain at the next turn boundary regardless of whether background subagents are still running. Background subagents are independent execution contexts — their presence should not block the main agent from accepting new direction from the user.

A reasonable fix would be either:

* Remove the `this.hasActiveBackgroundWork()` short-circuit in `enqueueItem` — let `processQueue()` start, and rely on the main agent's own turn boundaries to determine when to actually pull from the queue, OR
* Have `enqueueItem` always call `processQueue()` (or `emitDeferredSessionIdleIfReady()`) and let `processQueuedItems`'s own internal isProcessing/state checks decide what to do.

## Workarounds (in order of usefulness)

**1. Submit another message** ← this is the cargo-cult workaround that *appears* to work, and is what users naturally discover. It does not actually fix the queue state — it just buys time until a bg agent completes. Until a proper fix lands, this is what most users will reach for. **It is the path of least friction but is fundamentally a placebo; the real driver is the eventual bg-agent completion.**

2. **Wait it out.** Confirmed: the queue *does* drain automatically once both `idleDeferredByBackgroundWork` clears AND `hasActiveBackgroundWork()` returns false. For finite well-behaved bg agents, this works but feels unresponsive — minutes of waiting is common.

3. **Cancel current activity with Esc once.** This aborts in-flight work via the AbortController, which ultimately flushes through the same completion-callback path and drains the queue. Side effect: you lose whatever the main agent was doing.

4. **Copy-paste manually.** Select the text of the queued message from the terminal, double-Esc to drop the queue entry (this is destructive — confirmed by changelog 1.0.40: *"Ctrl+C and double-Esc remove pending queued messages one at a time"*), then re-submit when you see a clean idle state.

5. **`ctrl+q` is NOT a dequeue.** The status-bar hint `ctrl+q enqueue` that appears during active execution is one-directional — it adds to the queue, it cannot remove from it. We tested this empirically in this investigation; the only `"ctrl+q"` string literal in the minified bundle is the keybind label inside the `enqueueHint` status renderer.

## Severity

Medium. The bug doesn't lose data (messages eventually drain), but it severely degrades the responsiveness of long-running agentic sessions, especially autopilot mode with frequent bg-agent dispatch. In one of my real m2a sessions over a 12-hour span, this caused approximately 18 of 47 turns (~38%) to have NULL assistant responses corresponding to messages that I had to retype because they appeared to be stuck.

## Environment

- OS: Windows 11
- Terminal: Windows Terminal
- CLI: 1.0.49-1 (npm package `@github/copilot`)
- Model in use during investigation: Claude Opus 4.7 (1M context)

## Reachability

I'm filing this from my personal account because GitHub Enterprise Managed User policy blocks my work account from posting to this repo. If maintainers want to follow up directly, my work GitHub login is `samallon`. Feel free to ping there if any of the diagnostic detail needs clarification or if a different set of session-store events would help isolate the drain failure.

## Related issues

I searched the repo before filing and found no exact match, but these are adjacent and may inform triage:

- #2263 (closed) "FLEET: session.idle fires before all sub-agents complete" — this bug is plausibly a side-effect of the fix for that one. The fix introduced `idleDeferredByBackgroundWork` and the `hasActiveBackgroundWork()` guard, which is exactly the guard now stranding user messages.
- #2055 (open) "Queued messages cannot be cleared without canceling running job" — adjacent dequeue UX gap; both reflect the same underlying primitive (queue control while bg work is in flight) being underdeveloped.
- #2025 (open) "True non-blocking message queue for autopilot/multi-turn agent sessions" — feature request for an explicit "after-task" queue. If a proper drain semantic is added, the design space is shared with that proposal.
- #1857 (open) "Allow users to cancel or remove enqueued messages" — same dequeue-primitive gap.
- #2624 (open) "Consistent Enqueue shortcut to match VS Code Copilot's Alt + Enter" — adjacent keybinding/UX.
- #1780 (open) "Recover queued and pending prompts after agent crash" — adjacent durability concern.

@github/copilot@1.0.48 の minified app.js から始め、enqueueItem、processQueue、emitDeferredSessionIdleIfReady、および taskRegistry の完了コールバックを追跡します。90 秒のバックグラウンドタスクと 60 秒の同期 PowerShell sleep を使って重複実行を再現し、その後、サブエージェントの完了を必要とせず、キューに入れられたメッセージが次のターン境界で処理されることを確認します。

索引モデルが issue の本文から書いたものです。