[Error] potential listener LEAK — terminal/decorationAddon

[vs-code-engineering]

Error Message

unhandlederror-potential listener LEAK detected, popular

Stack Trace

    at a.create (./src/vs/base/common/event.ts:1062:14)
    at Rae._event [as onWillShutdown] (./src/vs/base/common/event.ts:1263:33)
    at new BUe (./src/vs/workbench/contrib/terminal/browser/xterm/decorationAddon.ts:89:0)
    at a._createInstance (./src/vs/platform/instantiation/common/instantiationService.ts:162:17)
    at a.createInstance (./src/vs/platform/instantiation/common/instantiationService.ts:128:17)
    at new MP (./src/vs/workbench/contrib/terminal/browser/xterm/xtermTerminal.ts:305:0)
    at a._createInstance (./src/vs/platform/instantiation/common/instantiationService.ts:162:17)
    at a.createInstance (./src/vs/platform/instantiation/common/instantiationService.ts:128:17)
    at xC.createDetachedTerminal (./src/vs/workbench/contrib/terminal/browser/terminalService.ts:1098:0)

Raw Stack Trace (minified)

Error     at a.create (out/vs/sessions/sessions.desktop.main.js:403:18384)     at Rae._event [as onWillShutdown] (out/vs/sessions/sessions.desktop.main.js:405:1891)     at new BUe (out/vs/sessions/sessions.desktop.main.js:3414:4739)     at a._createInstance (out/vs/sessions/sessions.desktop.main.js:1466:2000)     at a.createInstance (out/vs/sessions/sessions.desktop.main.js:1466:1440)     at new MP (out/vs/sessions/sessions.desktop.main.js:3415:8744)     at a._createInstance (out/vs/sessions/sessions.desktop.main.js:1466:2000)     at a.createInstance (out/vs/sessions/sessions.desktop.main.js:1466:1440)     at xC.createDetachedTerminal (out/vs/sessions/sessions.desktop.main.js:3560:55048)

Details

Property	Value
Bucket ID	cb64cc42-18ec-cb83-d4d6-13b8c55127e0
Version	1.117.0-insider
Commit	98b4d3e2
Last Seen	Thu Apr 16 2026 22:22:58 GMT+0000 (Coordinated Universal Time)
Platforms	Mac, Windows
Product	Agents

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

🔍 Error Investigation

Summary

Each DecorationAddon instance registers a listener on lifecycleService.onWillShutdown (line 89 of decorationAddon.ts). In the Agents product, many detached terminals are created via chatTerminalCommandMirror.ts for rendering terminal command output in chat. When the accumulated listener count on onWillShutdown exceeds the global leak warning threshold of 175, the leak detection fires this error. This is a latent issue amplified by high detached-terminal volume in the Agents product.

Culprit Commit

No single culprit commit was identified. The onWillShutdown listener in DecorationAddon has been present for a long time. The error manifests because the Agents product creates many more detached terminals than the standard VS Code workbench (each chat tool invocation creates a ChatTerminalOutputRenderable → createDetachedTerminal → XtermTerminal → DecorationAddon). The bucket history shows hits across versions 1.114–1.117, first appearing April 11, 2026, consistent with increasing Agents usage rather than a specific code change.

Code Flow

sequenceDiagram
    participant Agent as Chat Agent Tool
    participant Mirror as ChatTerminalOutputRenderable
    participant TermSvc as TerminalService
    participant Xterm as XtermTerminal
    participant Deco as DecorationAddon
    participant Lifecycle as LifecycleService

    Agent->>Mirror: render terminal output
    Mirror->>TermSvc: createDetachedTerminal()
    TermSvc->>Xterm: new XtermTerminal()
    Xterm->>Deco: new DecorationAddon()
    Deco->>Lifecycle: onWillShutdown(→ disposeAllDecorations)
    Note over Lifecycle: ⚠️ Listener #176 added<br/>exceeds threshold (175)
    Lifecycle-->>Deco: 💥 ListenerLeakError<br/>"potential listener LEAK detected, popular"

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Affected Files

File	Role
src/vs/workbench/contrib/terminal/browser/xterm/decorationAddon.ts:89	Crash site — registers per-instance onWillShutdown listener
src/vs/workbench/contrib/terminal/browser/chatTerminalCommandMirror.ts:611	Creates many detached terminals for chat output rendering
src/vs/workbench/contrib/terminal/browser/terminalService.ts:1122	createDetachedTerminal() — entry point for detached terminal creation
src/vs/workbench/contrib/terminal/browser/xterm/xtermTerminal.ts:317	Instantiates DecorationAddon during xterm setup
src/vs/workbench/services/lifecycle/common/lifecycleService.ts:23	_onWillShutdown emitter (no custom threshold)
src/vs/base/common/event.ts:1028	Leak detection logic (threshold 175)

Repro Steps

1. Open the Agents product (VS Code Sessions)
2. Start a chat session that uses terminal tools (e.g., run many shell commands via an agent)
3. Execute 175+ terminal tool invocations in a single session — each creates a detached terminal with a DecorationAddon
4. Observe the "potential listener LEAK detected" error in telemetry

The error is deterministic once the terminal count crosses the threshold. The exact count depends on other onWillShutdown listeners already registered.

Fix Proposal

Fix location: src/vs/workbench/contrib/terminal/browser/xterm/decorationAddon.ts → constructor / src/vs/workbench/contrib/terminal/browser/terminalService.ts → _ensureDetachedTerminalListeners()

Approach: Apply the same consolidation pattern already used for theme/config listeners in _ensureDetachedTerminalListeners() (terminalService.ts:1150). Instead of each DecorationAddon registering its own onWillShutdown listener, register a single onWillShutdown listener in _ensureDetachedTerminalListeners() that iterates over all detached terminals and calls _disposeAllDecorations() on each. This keeps one listener regardless of how many detached terminals exist. Alternatively, DecorationAddon could accept a flag indicating it's in a detached context and skip onWillShutdown registration entirely, since detached terminals are disposed explicitly and don't need shutdown cleanup.

Recommended Owner

@Tyriar — terminal area owner, owns DecorationAddon, XtermTerminal, and the _ensureDetachedTerminalListeners consolidation pattern in terminalService.ts

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