[Error] unhandlederror-potential listener LEAK detected, popular — textModel / chatEditingModifiedNotebookEntry

[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 Oje._event [as onDidChange] (./src/vs/base/common/event.ts:1262:33)
    at new pp (./src/vs/editor/common/model/textModel.ts:408:52)
    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 JZe._createModelData (./src/vs/editor/common/services/modelService.ts:310:54)
    at JZe.createModel (./src/vs/editor/common/services/modelService.ts:444:20)
    at E0e.provideTextContent (./src/vs/workbench/contrib/notebook/browser/notebook.contribution.ts:409:0)
    at async $Ze.ensureResolvedTextModelContent (./src/vs/workbench/services/textmodelResolver/common/textModelResolverService.ts:192:7)
    at async $Ze.doCreateReferencedObject (./src/vs/workbench/services/textmodelResolver/common/textModelResolverService.ts:65:3)
    at async $Se.acquire (./src/vs/base/common/lifecycle.ts:727:18)
    at async jZe.createModelReference (./src/vs/workbench/services/textmodelResolver/common/textModelResolverService.ts:242:9)
    at async hE.resolveCellModel (./src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingModifiedNotebookEntry.ts:1005:0)

Raw Stack Trace (minified)

Error     at a.create (out/vs/workbench/workbench.desktop.main.js:403:18382)     at Oje._event [as onDidChange] (out/vs/workbench/workbench.desktop.main.js:405:1807)     at new pp (out/vs/workbench/workbench.desktop.main.js:668:38398)     at a._createInstance (out/vs/workbench/workbench.desktop.main.js:1890:2000)     at a.createInstance (out/vs/workbench/workbench.desktop.main.js:1890:1440)     at JZe._createModelData (out/vs/workbench/workbench.desktop.main.js:1795:2490)     at JZe.createModel (out/vs/workbench/workbench.desktop.main.js:1796:898)     at E0e.provideTextContent (out/vs/workbench/workbench.desktop.main.js:2992:18869)     at async $Ze.ensureResolvedTextModelContent (out/vs/workbench/workbench.desktop.main.js:1771:29274)     at async $Ze.doCreateReferencedObject (out/vs/workbench/workbench.desktop.main.js:1771:28023)     at async $Se.acquire (out/vs/workbench/workbench.desktop.main.js:403:3866)     at async jZe.createModelReference (out/vs/workbench/workbench.desktop.main.js:1771:30166)     at async hE.resolveCellModel (out/vs/workbench/workbench.desktop.main.js:3469:15036)

Details

Property	Value
Bucket ID	d9bac165-5c08-a538-51c7-610c094f8c70
Version	1.116.0
Commit	560a9dba
Last Seen	Mon Apr 20 2026 23:59:05 GMT+0000 (Coordinated Universal Time)
Platforms	Mac, Windows, Linux
Product	VSCode

---

This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

Stable Release Anomaly Details

• Anomaly type: new
• Previous stable release: 1.115.0
• Current stable release: 1.116.0
• Last release without this error: 1.116.0-insider (47add6df)
• First release with this error: 1.116.0-insider (1dfb3b9d)
• Commit range: compare 47add6df...1dfb3b9d

Key Stack Frames (deminified)

#	Source	Function
1	event.ts:1062	create
2	event.ts:1262	_event (onDidChange)
3	textModel.ts:408	(constructor)
4	modelService.ts:310	_createModelData
5	modelService.ts:444	createModel
6	notebook.contribution.ts:409	provideTextContent
7	textModelResolverService.ts	ensureResolvedTextModelContent / createModelReference
8	chatEditingModifiedNotebookEntry.ts:1005	resolveCellModel

Analysis

This is a new listener leak error appearing first in 1.116.0 stable (11,976 users across Windows, Linux, Mac). The leak occurs when the chat editing system resolves notebook cell models: chatEditingModifiedNotebookEntry.resolveCellModel → notebook.contribution.provideTextContent → modelService.createModel → textModel.onDidChange triggers "popular" listener threshold. The commit range spans April 10–13 (47add6d → 1dfb3b9).

Generated by errors-stable-scan · ● 24.3M · ◷

[vs-code-engineering]

🔍 Error Investigation

Summary

The _languageConfigurationService.onDidChange emitter accumulates too many listeners (threshold: 175) because notebook chat editing creates a TextModel per cell via resolveCellModel, and each TextModel subscribes to this shared emitter. With notebooks containing many cells and other open editors, the total listener count exceeds the threshold. The "popular" classification indicates no single caller dominates — this is an overly popular emitter scenario, though the notebook editing code is the largest contributor. A race condition in createInsertedCellDiffInfo (double concurrent call to resolveCellModel for the same URI) exacerbates the issue by creating duplicate model references.

Culprit Commit

Field	Value
Commit	6cf846ce
Author	@roblourens
PR	#309093
Message	Don't remove model ref on disposal
Why	Removes this._session = undefined! from ChatResponseModel.dispose(), which prevents session references from being cleared. This can cause a retention chain: disposed ChatResponseModel → ChatModel → editing sessions → notebook entries → TextModel references → listener subscriptions on _languageConfigurationService.onDidChange. Combined with the already-present notebook cell model accumulation pattern, this pushes listener counts past the 175 threshold. The error first appears in build 1dfb3b9d (Apr 13), 3 days after this commit (6cf846ce, Apr 10), within the exact regression range (47add6df...1dfb3b9d).

Note: The underlying listener accumulation pattern in resolveCellModel is pre-existing (present in 1.115.0). This commit likely made the regression manifest by increasing the lifetime of retained TextModels.

Code Flow

sequenceDiagram
    participant CE as ChatEditingModifiedNotebookEntry
    participant RC as resolveCellModel()
    participant TMS as TextModelResolverService
    participant NB as notebook.contribution.ts
    participant MS as ModelService
    participant TM as TextModel (constructor)
    participant LCS as LanguageConfigurationService.onDidChange

    CE->>RC: resolveCellModel(cellURI) × N cells
    RC->>TMS: createModelReference(cell.uri)
    TMS->>NB: provideTextContent(resource)
    NB->>MS: createModel(bufferFactory, languageSelection, resource)
    MS->>TM: new TextModel(...)
    TM->>LCS: this._register(onDidChange(e => ...))
    Note over LCS: ⚠️ Each TextModel adds 1 listener.<br/>With N cells × 2 (modified + original),<br/>plus other editors, exceeds 175 threshold.
    LCS-->>TM: 💥 ListenerLeakError("popular")<br/>when count ≥ 175
```

### Affected Files

| File | Role |
|------|------|
| `src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingModifiedNotebookEntry.ts` | Root cause — creates TextModels per cell via `resolveCellModel()`, race condition in `createInsertedCellDiffInfo` (lines 755+769 call same URI concurrently) |
| `src/vs/workbench/contrib/chat/common/model/chatModel.ts` | Culprit commit — `ChatResponseModel.dispose()` no longer clears `_session`, retaining editing session references |
| `src/vs/editor/common/model/textModel.ts:408` | Crash site — TextModel constructor subscribes to `_languageConfigurationService.onDidChange` |
| `src/vs/editor/common/languages/languageConfigurationRegistry.ts:64` | Popular emitter — `onDidChange` Emitter has no custom `leakWarningThreshold` override |
| `src/vs/workbench/contrib/notebook/browser/notebook.contribution.ts:409` | Data flow — `provideTextContent()` creates TextModel for each notebook cell URI |
| `src/vs/base/common/event.ts:1028-1034` | Detection — `LeakageMonitor.check()` fires "popular" error when ratio ≤ 0.3 |

### Repro Steps

1. Open VS Code with several files/editors already open (to have existing TextModels)
2. Open a notebook (`.ipynb`) with 50+ cells
3. Start a Copilot Edits session that modifies the notebook
4. The chat editing system calls `initializeModelsFromDiff()` which resolves TextModels for all cells (2 per unchanged cell: modified + original)
5. Each TextModel subscribes to `_languageConfigurationService.onDidChange` — with 50 cells that's ~100+ new listeners, exceeding the 175 threshold when combined with existing editors
6. The "potential listener LEAK detected, popular" error fires

### Fix Proposal

Two issues need to be addressed:

**1. Fix the race condition in `createInsertedCellDiffInfo`** (quick win)
**Fix location**: `chatEditingModifiedNotebookEntry.ts` → `createInsertedCellDiffInfo()`
**Approach**: Lines 755 and 769 both call `this.resolveCellModel(cell.uri)` concurrently for the same URI. The second call starts before the first completes, so both miss the `cellTextModelMap` cache and create duplicate model references. Fix by storing the promise from the first call and reusing it:
```
const cellModelPromise = this.resolveCellModel(cell.uri);
cellModelPromise.then(modifiedModel => { ... });
return { modifiedModel: new ObservablePromise(cellModelPromise), ... };

Loading

2. Use a promise-based cache to prevent all concurrent duplicate resolutions
Fix location: chatEditingModifiedNotebookEntry.ts → resolveCellModel()
Approach: Change cellTextModelMap from ResourceMap<ITextModel> to ResourceMap<Promise<ITextModel>> so the first call sets the promise immediately, and concurrent calls await the same promise instead of creating duplicate model references.

3. (Optional) Increase leak threshold on the popular emitter
Fix location: languageConfigurationRegistry.ts → LanguageConfigurationServiceImpl
Approach: Since _languageConfigurationService.onDidChange is legitimately listened to by every TextModel, consider setting a higher leakWarningThreshold (e.g., 500) on this specific emitter to avoid false positives.

Recommended Owner

@DonJayamanne — area owner for "Copilot Edits support for Notebook" per working areas. The primary fix locations are in chatEditingModifiedNotebookEntry.ts which is in Don's ownership area.

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