[Error] potential listener LEAK — contextKeyService

[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 $Pe._event (./src/vs/base/common/event.ts:1263:33)
    at b_i._updateParentChangeListener (./src/vs/platform/contextkey/browser/contextKeyService.ts:497:0)
    at new b_i (./src/vs/platform/contextkey/browser/contextKeyService.ts:481:0)
    at $Pe.createScoped (./src/vs/platform/contextkey/browser/contextKeyService.ts:310:0)
    at $9.renderTemplate (./src/vs/sessions/contrib/changes/browser/changesViewRenderer.ts:203:75)
    at tTt.renderTemplate (./src/vs/base/browser/ui/tree/objectTree.ts:122:29)
    at a.renderTemplate (./src/vs/base/browser/ui/tree/abstractTree.ts:431:0)
    at (anonymous) (./src/vs/base/browser/ui/list/listWidget.ts:1273:35)
    at vci.renderTemplate (./src/vs/base/browser/ui/list/listWidget.ts:1273:24)
    at Mxt.alloc (./src/vs/base/browser/ui/list/rowCache.ts:40:33)
    at GR.insertItemInDOM (./src/vs/base/browser/ui/list/listView.ts:962:30)
    at (anonymous) (./src/vs/base/browser/ui/list/listView.ts:931:10)
    at makeChanges (./src/vs/base/browser/ui/list/rowCache.ts:69:3)
    at GR.render (./src/vs/base/browser/ui/list/listView.ts:922:13)
    at GR.onScroll (./src/vs/base/browser/ui/list/listView.ts:1165:8)
    at E._deliver (./src/vs/base/common/event.ts:1355:12)
    at E._deliverQueue (./src/vs/base/common/event.ts:1366:8)
    at E.fire (./src/vs/base/common/event.ts:1390:8)
    at _ye.value (./src/vs/base/browser/ui/scrollbar/scrollableElement.ts:220:18)

Raw Stack Trace (minified)

Error     at a.create (out/vs/sessions/sessions.desktop.main.js:403:18384)     at $Pe._event (out/vs/sessions/sessions.desktop.main.js:405:1891)     at b_i._updateParentChangeListener (out/vs/sessions/sessions.desktop.main.js:1437:38167)     at new b_i (out/vs/sessions/sessions.desktop.main.js:1437:37791)     at $Pe.createScoped (out/vs/sessions/sessions.desktop.main.js:1437:35531)     at $9.renderTemplate (out/vs/sessions/sessions.desktop.main.js:4115:12130)     at tTt.renderTemplate (out/vs/sessions/sessions.desktop.main.js:861:18655)     at a.renderTemplate (out/vs/sessions/sessions.desktop.main.js:860:17232)     at out/vs/sessions/sessions.desktop.main.js:728:2084     at Array.map (<anonymous>)     at vci.renderTemplate (out/vs/sessions/sessions.desktop.main.js:728:2075)     at Mxt.alloc (out/vs/sessions/sessions.desktop.main.js:685:5675)     at GR.insertItemInDOM (out/vs/sessions/sessions.desktop.main.js:685:21720)     at out/vs/sessions/sessions.desktop.main.js:685:21230     at Mxt.transact (out/vs/sessions/sessions.desktop.main.js:685:5912)     at GR.render (out/vs/sessions/sessions.desktop.main.js:685:21090)     at GR.onScroll (out/vs/sessions/sessions.desktop.main.js:685:27657)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3150)     at E._deliverQueue (out/vs/sessions/sessions.desktop.main.js:405:3239)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3574)     at _ye.value (out/vs/sessions/sessions.desktop.main.js:483:36808)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3150)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3469)     at $_._setState (out/vs/sessions/sessions.desktop.main.js:414:21424)     at $_.setScrollDimensions (out/vs/sessions/sessions.desktop.main.js:414:19430)     at oF.setScrollDimensions (out/vs/sessions/sessions.desktop.main.js:483:39181)     at GR.layout (out/vs/sessions/sessions.desktop.main.js:685:20360)     at uui.layout (out/vs/sessions/sessions.desktop.main.js:728:10167)     at iM.layout (out/vs/sessions/sessions.desktop.main.js:860:56372)     at _ye.value (out/vs/sessions/sessions.desktop.main.js:4121:28357)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3150)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3469)     at Object.layout (out/vs/sessions/sessions.desktop.main.js:1299:113533)     at co (out/vs/sessions/sessions.desktop.main.js:1299:112038)     at Object.G [as runAndSubscribe] (out/vs/sessions/sessions.desktop.main.js:403:15951)     at XMe.create (out/vs/sessions/sessions.desktop.main.js:1299:112073)     at jM.createModalEditorPart (out/vs/sessions/sessions.desktop.main.js:1299:122423)     at otn (out/vs/sessions/sessions.desktop.main.js:1299:56827)     at whe (out/vs/sessions/sessions.desktop.main.js:1299:55243)     at a.invokeFunction (out/vs/sessions/sessions.desktop.main.js:1466:944)     at bN.resolveUntypedInputAndGroup (out/vs/sessions/sessions.desktop.main.js:1347:51939)     at bN.resolveEditor (out/vs/sessions/sessions.desktop.main.js:1347:52185)     at Rhe.openEditor (out/vs/sessions/sessions.desktop.main.js:1347:44707)     at bqe._openFileItem (out/vs/sessions/sessions.desktop.main.js:4121:30313)     at _ye.value (out/vs/sessions/sessions.desktop.main.js:4121:23696)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3150)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3469)     at Mui._open (out/vs/sessions/sessions.desktop.main.js:861:61260)     at Mui.onPointer (out/vs/sessions/sessions.desktop.main.js:861:60943)     at _ye.value (out/vs/sessions/sessions.desktop.main.js:861:60075)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3150)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3469)     at _ye.value (out/vs/sessions/sessions.desktop.main.js:403:11566)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3150)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3469)     at dui.onViewPointer (out/vs/sessions/sessions.desktop.main.js:685:48200)     at dui.onViewPointer (out/vs/sessions/sessions.desktop.main.js:860:47487)     at _ye.value (out/vs/sessions/sessions.desktop.main.js:403:11923)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3150)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3469)     at _ye.value (out/vs/sessions/sessions.desktop.main.js:403:11566)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3150)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3469)     at HTMLDivElement.o (out/vs/sessions/sessions.desktop.main.js:483:63361)

Details

Property	Value
Bucket ID	05c8d306-2d3a-1046-784d-ea0c1c891158
Version	1.117.0-insider
Commit	98b4d3e2
Last Seen	Wed Apr 15 2026 22:47:43 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 changesViewRenderer.ts tree renderer creates a ScopedContextKeyService (via createScoped()) in its renderTemplate() method for each pooled template. Each scoped service subscribes to the parent context key service's onDidChangeContext emitter. When the changes tree view has enough visible rows (or templates accumulate in the row cache), the parent emitter's listener count exceeds the leak-detection threshold, triggering the "potential listener LEAK detected, popular" error. This is a pre-existing issue across multiple Agents Insider builds — the specific error bucket for commit 98b4d3e2 appeared because minified symbol names changed, creating a new stack hash. The telemetry search shows 16+ similar listener leak buckets across the Agents product spanning multiple versions.

Culprit Commit

There is no single culprit commit that introduced this regression. The listener leak pattern has existed since the changesViewRenderer.ts file was created. The error appeared as a new telemetry bucket on commit 98b4d3e2 due to a combination of:

1. PR #308897 (by @bryanchen-d, merged Apr 14) enriched ListenerLeakError with kind and listenerCount fields, changing the error message format from "potential listener LEAK detected" to "potential listener LEAK detected, popular" — creating new error buckets for pre-existing leaks
2. Minified symbol name changes between builds cause different stack trace hashes, splitting the same underlying leak into multiple buckets

The file was introduced/organized by @lszomoru in PR #307903 and later touched by @sandy081 in PR #308189.

Code Flow

sequenceDiagram
    participant User as User Action (click/scroll)
    participant ListView as ListView.render()
    participant RowCache as RowCache.alloc()
    participant Renderer as ChangesTreeRenderer.renderTemplate()
    participant CKS as ContextKeyService.createScoped()
    participant Parent as Parent CKS._onDidChangeContext

    User->>ListView: scroll / layout
    ListView->>RowCache: alloc new template
    RowCache->>Renderer: renderTemplate(container)
    Note over Renderer: ⚠️ Root cause: creates scoped CKS<br/>per template (line 203)
    Renderer->>CKS: createScoped(actionBarContainer)
    CKS->>Parent: subscribe to onDidChangeContext
    Note over Parent: 💥 Listener count exceeds threshold<br/>"potential listener LEAK detected, popular"

Loading

Affected Files

File	Role
src/vs/sessions/contrib/changes/browser/changesViewRenderer.ts	Root cause — creates scoped context key service per template (line 203)
src/vs/platform/contextkey/browser/contextKeyService.ts	ScopedContextKeyService._updateParentChangeListener() subscribes to parent (line 497)
src/vs/base/common/event.ts	Leak detection in LeakageMonitor.check() (line 1008), Emitter.event getter (line 1261)

Repro Steps

1. Open VS Code Agents (Insiders)
2. Start a session that produces many file changes (e.g., an agent coding session with 50+ modified files)
3. Open the Changes view in the sidebar
4. Scroll through the file list — as templates are allocated from the row cache, each one creates a ScopedContextKeyService subscribing to the parent
5. Once the listener count on the parent context key service's onDidChangeContext emitter exceeds the threshold (typically 200), the error is thrown
6. The error may also trigger during layout events when opening files from the changes view (as seen in the full stack trace)

Fix Proposal

Fix location: src/vs/sessions/contrib/changes/browser/changesViewRenderer.ts → renderTemplate()

Approach: The recommended fix is to not create a scoped context key service per template. Instead:

1. Option A (preferred): Move the createScoped call from renderTemplate() to renderElement() / disposeElement(), storing the scoped service in elementDisposables instead of templateDisposables. This way, each scoped service is disposed when the element is recycled, keeping the active listener count bounded to the number of visible rows rather than the total template pool size.

2. Option B: Share a single ScopedContextKeyService across all templates by creating it once in the renderer constructor and updating its context values in renderElement(). This is more efficient but requires careful management of the context key bindings.

3. Option C (quick fix): Increase the leak monitoring threshold on the parent context key service's _onDidChangeContext emitter. This suppresses the error but doesn't fix the underlying accumulation.

Recommended Owner

@lszomoru — primary author of the sessions changes view code (PR #307903, PR #308189 area)

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