[Error] unhandlederror-potential listener LEAK detected, popular

[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 hte._event (./src/vs/base/common/event.ts:1263:33)
    at new BUe (./src/vs/workbench/contrib/terminal/browser/xterm/decorationAddon.ts:85:0)
    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 hte._event (out/vs/sessions/sessions.desktop.main.js:405:1891)     at new BUe (out/vs/sessions/sessions.desktop.main.js:3414:4436)     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	f8e4be6c-eb36-f9ac-3c53-05a898bb38a5
Version	1.117.0-insider
Commit	98b4d3e2
Last Seen	Fri Apr 17 2026 21:39:32 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]

🔍 Error Investigation

Summary

The DecorationAddon constructor registers listeners on global singleton services (themeService.onDidColorThemeChange, configurationService.onDidChangeConfiguration, lifecycleService.onWillShutdown) for every detached terminal instance. When the Agents product creates many detached terminals concurrently, these per-instance listeners accumulate on the global emitter and trigger the "potential listener LEAK detected, popular" error. PR #310157 fixed the same class of leak in XtermTerminal by skipping global listeners for detached terminals, but the DecorationAddon was not updated to match.

Culprit Commit

Field	Value
Commit	15806fe5e0 (via merge)
Author	@meganrogge
PR	#310157
Message	Fix listener leak in terminal tool progress parts, execute strategies, and detached terminals
Why	This PR added a detached guard to XtermTerminal to skip global service listeners, and centralized those listeners in TerminalService._ensureDetachedTerminalListeners(). However, it did not apply the same treatment to DecorationAddon, which is created unconditionally at xtermTerminal.ts:317 and still registers its own global listeners in its constructor.

Related issue: #310046 was the same listener leak in xtermTerminal.ts (fixed by #310157). This issue is the residual leak from DecorationAddon that the fix missed.

Code Flow

sequenceDiagram
    participant Agent as Agents Product
    participant TS as TerminalService
    participant XT as XtermTerminal
    participant DA as DecorationAddon
    participant Theme as ThemeService (global)

    Agent->>TS: createDetachedTerminal()
    TS->>XT: new XtermTerminal({detached: true})
    Note over XT: ✅ Skips global listeners<br/>(line 284: if !options.detached)
    XT->>DA: createInstance(DecorationAddon)
    Note over DA: ⚠️ Root cause:<br/>No detached guard — always<br/>registers global listeners
    DA->>Theme: onDidColorThemeChange (line 85)
    Note over Theme: 💥 Listener count exceeds<br/>threshold → ListenerLeakError

Loading

Affected Files

File	Role
src/vs/workbench/contrib/terminal/browser/xterm/decorationAddon.ts	Root cause — constructor (lines 75-89) registers global service listeners without detached guard
src/vs/workbench/contrib/terminal/browser/xterm/xtermTerminal.ts	Creates DecorationAddon unconditionally at line 317 (no detached skip)
src/vs/workbench/contrib/terminal/browser/terminalService.ts	_ensureDetachedTerminalListeners() centralizes theme/config for XtermTerminal but not for DecorationAddon
src/vs/base/common/event.ts	Crash site — ListenerLeakError thrown at line 1074 when listener threshold exceeded

Repro Steps

1. Open VS Code Insiders with the Agents product enabled
2. Trigger multiple concurrent agent sessions that each create a detached terminal (e.g., run several tool-use requests in parallel)
3. Each createDetachedTerminal() call creates an XtermTerminal → DecorationAddon, each registering its own themeService.onDidColorThemeChange listener
4. After ~200 detached terminals are created (threshold depends on the emitter's leak detection config), the listener leak error fires

The error is deterministic once enough detached terminals exist concurrently. Likelihood increases with Agents workloads that create many parallel terminals.

Fix Proposal

Apply the same pattern used in XtermTerminal (PR #310157) to DecorationAddon:

Option A — Skip global listeners for detached terminals in DecorationAddon:

• Fix location: src/vs/workbench/contrib/terminal/browser/xterm/decorationAddon.ts → constructor
• Approach: Accept a detached flag (or pass the IXtermTerminalOptions through). When detached is true, skip registering configurationService.onDidChangeConfiguration, themeService.onDidColorThemeChange, and lifecycleService.onWillShutdown in the constructor (lines 75-89). Instead, forward these updates from the centralized listener in TerminalService._ensureDetachedTerminalListeners(), similar to how XtermTerminal.updateConfig() and XtermTerminal.updateTheme() are already forwarded.

Option B — Skip DecorationAddon entirely for detached terminals:

• Fix location: src/vs/workbench/contrib/terminal/browser/xterm/xtermTerminal.ts → line 317
• Approach: If detached terminals don't need shell integration decorations (command markers, etc.), wrap the DecorationAddon creation in an if (!options.detached) guard, similar to the guard at line 284. This is simpler but only valid if detached terminals never need decorations.

Recommended Owner

@meganrogge — authored PR #310157 that fixed the same leak class in XtermTerminal; this is a follow-up gap in that fix.

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