agents: fix spinner stuck after session completes

[roblourens]

Problem

Completed agent host sessions show a perpetual spinner in the Sessions list. The session finishes successfully but the UI never transitions away from the "in progress" state.

Root cause

There is a race between AgentHostStateManager's _summaryNotifyScheduler (100 ms debounce) and the call to removeSession triggered by _maybeEvictIdleSession.

1. SessionTurnComplete is processed → endTurn produces a new summary with status = Idle → session is added to _dirtySummaries → _summaryNotifyScheduler.schedule() (fires in 100 ms).
2. Within that 100 ms window: the client calls unsubscribe, _maybeEvictIdleSession sees activeTurn === undefined and no remaining subscribers → calls removeSession.
3. removeSession clears _sessionStates, _lastNotifiedSummaries, and _dirtySummaries.
4. The scheduler fires 100 ms later, finds nothing dirty → no SessionSummaryChanged notification is ever emitted.
5. The client's _handleSessionSummaryChanged never receives status = Idle → spinner stays.

Evidence from live IPC/agent-host logs: SessionTurnComplete envelopes arrive for all three sessions, followed ~250 ms later by "Action for unknown session" warnings — confirming the session was evicted before the flush.

Fix

removeSession: before clearing state maps, synchronously flush any pending summary notification. This guarantees clients see the final status even when a session is evicted within the debounce window.

deleteSession (permanent removal): drop the session from _dirtySummaries before calling removeSession. The forthcoming SessionRemoved notification supersedes any pending summary diff, so we preserve the existing invariant that permanently-deleted sessions don't emit spurious SessionSummaryChanged events.

Testing

Added a regression test to agentHostStateManager.test.ts:

• Start a turn → let the scheduler fire so _lastNotifiedSummaries records status = InProgress
• Complete the turn → call removeSession before the 100 ms debounce fires
• Assert SessionSummaryChanged { status: Idle } was emitted synchronously inside removeSession

All 31 existing tests continue to pass.

(Written by Copilot)

[github-actions]

Screenshot Changes

Base: 724775b9 Current: d7dafffe

Changed (6)

chat/input/chatInput/WithArtifacts/Light

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chat/input/chatInput/WithFileChanges/Light

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chat/input/chatInput/WithTodosAndFileChanges/Dark

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chat/input/chatInput/WithArtifactsAndFileChanges/Light

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chat/widget/chatWidget/MultiTurn/Light

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[Copilot]

Pull request overview

Fixes a race in the agent-host sessions process where a session can be evicted during the debounced summary-notification window, causing clients to miss the final status = Idle update and leaving the Sessions list spinner stuck “in progress”.

Changes:

• Flushes any pending SessionSummaryChanged notification synchronously during removeSession teardown, ensuring the final status reaches clients even if eviction happens before the debounce fires.
• Ensures deleteSession suppresses any pending summary diff so clients don’t see a last-moment SessionSummaryChanged right before SessionRemoved.
• Adds a regression test covering eviction within the debounce window after a turn completes.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/agentHostStateManager.ts	Flush pending summary notifications on eviction; refactors flush logic into a helper; suppresses pending diffs on delete.
src/vs/platform/agentHost/test/node/agentHostStateManager.test.ts	Adds a regression test verifying removeSession synchronously emits the final status = Idle summary change.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/platform/agentHost/node/agentHostStateManager.ts:417

• For consistency with the rest of AgentHostStateManager (which uses the protocol URI type alias for session identifiers), consider changing _flushSummaryNotificationFor(session: string) to _flushSummaryNotificationFor(session: URI). This avoids mixing raw string with the semantic URI type in this API surface.

	/**
	 * Emits a {@link NotificationType.SessionSummaryChanged} notification for
	 * `session` if its current summary differs from the last one sent to
	 * clients, then advances `_lastNotifiedSummaries` to the current summary.
	 *
	 * Does NOT remove `session` from `_dirtySummaries` — callers are
	 * responsible for that bookkeeping.
	 */
	private _flushSummaryNotificationFor(session: string): void {
		const state = this._sessionStates.get(session);

• Files reviewed: 2/2 changed files
• Comments generated: 2