[Unhandled Error] potential listener LEAK detected, popular — terminal/xtermTerminal / terminalService

[vs-code-engineering]

Error Message

potential listener LEAK detected, popular

Stack Trace

    at a.create (./src/vs/base/common/event.ts:1062:14)
    at ate._event (./src/vs/base/common/event.ts:1262:33)
    at new xP (./src/vs/workbench/contrib/terminal/browser/xterm/xtermTerminal.ts:289:0)
    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:403:18382)     at ate._event (out/vs/sessions/sessions.desktop.main.js:405:1807)     at new xP (out/vs/sessions/sessions.desktop.main.js:3443:8169)     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:3588:54929)

Details

Property	Value
Bucket ID	422a1210-9a5a-e865-fe80-14fbb05c3836
Version	1.116.0-insider
Commit	1dfb3b9d
Last Seen	Mon Apr 13 2026 22:41:40 GMT+0000 (Coordinated Universal Time)
Platforms	Mac, Windows, Linux
Product	Agents

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

Regression Details

• Product: Agents
• Regression type: new
• Last release without this error: 1.116.0-insider (fc6d18c3)
• First release with this error: 1.116.0-insider (e4f6e114)
• Commit range: fc6d18c3...e4f6e114

Generated by errors-agents-regression-scan · ● 4.6M · ◷

[vs-code-engineering]

🔍 Error Investigation

Summary

The error potential listener LEAK detected, popular is triggered when the onDidColorThemeChange event emitter on the theme service accumulates too many listeners (exceeding the leak detection threshold). Each XtermTerminal instance created via createDetachedTerminal registers a listener on this global service. In the Agents product, each terminal tool invocation creates a DetachedTerminalSnapshotMirror (or DetachedTerminalCommandMirror), each instantiating a full XtermTerminal. When an agent session runs many terminal commands, the cumulative listener count exceeds the threshold.

Culprit Commit

No single culprit commit was identified. This is a design-level issue: the DetachedTerminalSnapshotMirror constructor eagerly creates a full XtermTerminal (with all its global service listeners) for each terminal command output snapshot. The error first appeared in telemetry on April 10, 2026, likely due to growing Agents product usage pushing the listener count over the threshold. The error persists across all commits from 3d70aab9 (1.115.0) through 1dfb3b9d (1.116.0).

Related issue: #308254 (same root cause, different stack frame — listener leak from detached terminal links contribution).

Code Flow

sequenceDiagram
    participant Chat as ChatTerminalToolProgressPart
    participant Section as ChatTerminalToolOutputSection
    participant Snapshot as DetachedTerminalSnapshotMirror
    participant Service as TerminalService
    participant Xterm as XtermTerminal

    Chat->>Section: _renderSnapshotOutput(snapshot)
    Section->>Snapshot: new DetachedTerminalSnapshotMirror()
    Note over Snapshot: Constructor eagerly creates<br/>a full XtermTerminal
    Snapshot->>Service: createDetachedTerminal()
    Service->>Xterm: new XtermTerminal()
    Note over Xterm: Registers listeners on:<br/>• onDidColorThemeChange<br/>• onDidChangeConfiguration<br/>• onDidChangeLogLevel
    Note over Xterm: ⚠️ Root cause: Each instance<br/>adds listeners to global singletons
    Xterm-->>Service: xterm instance
    Note over Chat: After N tool invocations...
    Note over Xterm: 💥 ListenerLeakError thrown:<br/>"potential listener LEAK<br/>detected, popular"

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Affected Files

File	Role
src/vs/base/common/event.ts:1062	Crash site — Stacktrace.create() in leak detection
src/vs/base/common/event.ts:1262	Emitter subscription check triggers leak detection
src/vs/workbench/contrib/terminal/browser/xterm/xtermTerminal.ts:289	Root cause — registers onDidColorThemeChange listener per instance
src/vs/workbench/contrib/terminal/browser/chatTerminalCommandMirror.ts:611	Data producer — DetachedTerminalSnapshotMirror constructor eagerly creates XtermTerminal
src/vs/workbench/contrib/terminal/browser/chatTerminalCommandMirror.ts:413	Data producer — DetachedTerminalCommandMirror._getOrCreateTerminal() also creates detached terminals
src/vs/workbench/contrib/terminal/browser/terminalService.ts:1098	createDetachedTerminal entry point
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/toolInvocationParts/chatTerminalToolProgressPart.ts:1396	Creates DetachedTerminalSnapshotMirror per tool result

Repro Steps

1. Open VS Code Insiders (Agents product / sessions window)
2. Start an agent session that runs many terminal commands (e.g., a coding agent running build/test/lint commands)
3. As the agent runs 50+ terminal commands, each creates a DetachedTerminalSnapshotMirror to display the output
4. Each mirror creates a full XtermTerminal that registers listeners on global services
5. When the listener count on onDidColorThemeChange exceeds the threshold, the leak error is thrown
6. The error is more likely to trigger with longer agent sessions that produce more terminal outputs

Fix Proposal

Two complementary approaches:

1. Lazy terminal creation in DetachedTerminalSnapshotMirror (primary fix)
Fix location: src/vs/workbench/contrib/terminal/browser/chatTerminalCommandMirror.ts → DetachedTerminalSnapshotMirror constructor
Approach: Move the createDetachedTerminal call out of the constructor and into the attach() or render() method. The terminal should only be created when the snapshot is actually being displayed, not when the mirror object is constructed. This would also allow the terminal to be disposed when scrolled out of view in virtualized lists.

2. Lightweight rendering for snapshot output (longer-term)
Fix location: src/vs/workbench/contrib/terminal/browser/chatTerminalCommandMirror.ts → DetachedTerminalSnapshotMirror
Approach: For read-only snapshot terminals that display static output, consider using a lightweight rendering approach (e.g., a canvas or pre-rendered HTML) instead of a full XtermTerminal instance with all its service listeners. Snapshots don't need live theme change handling, configuration watching, or log level monitoring.

3. Increase listener threshold for detached terminals (quick mitigation)
Fix location: src/vs/workbench/contrib/terminal/browser/xterm/xtermTerminal.ts constructor
Approach: When creating XtermTerminal for detached/snapshot use, consider not registering on global service events, or unsubscribing when not attached to the DOM.

Recommended Owner

@meganrogge — area owner for terminal chat integration, authored the DetachedTerminalSnapshotMirror/DetachedTerminalCommandMirror classes and the createDetachedTerminal infrastructure (PRs #285985, #283906, #280442)

Generated by errors-investigate · ● 7.1M · ◷