[Error] unhandlederror-potential listener LEAK detected, popular — webview/webviewElement

[vs-code-engineering]

Error Message

unhandlederror-potential listener LEAK detected, popular

Stack Trace

    at a.create (./src/vs/base/common/event.ts:1068:14)
    at _Ie._event (./src/vs/base/common/event.ts:1272:33)
    at Object.start (./src/vs/workbench/contrib/webview/browser/webviewElement.ts:804:0)
    at r$e.loadResource (./src/vs/workbench/contrib/webview/browser/webviewElement.ts:794:0)

Raw Stack Trace (minified)

Error     at a.create (out/vs/workbench/workbench.desktop.main.js:403:18436)     at _Ie._event (out/vs/workbench/workbench.desktop.main.js:405:2009)     at Object.start (out/vs/workbench/workbench.desktop.main.js:5205:27934)     at r$e.loadResource (out/vs/workbench/workbench.desktop.main.js:5205:27837)

Details

Property	Value
Bucket ID	9c81d6ca-bb53-db80-91a5-89fe3ad53386
Version	1.118.0-insider
Commit	659d444a
Last Seen	Thu Apr 23 2026 23:58:09 GMT+0000 (Coordinated Universal Time)
Platforms	Mac, Windows, Linux
Product	VSCode

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

Regression Details

• Product: VSCode
• Regression type: new
• Last release without this error: 1.118.0-insider (40db337, Apr 22 17:06)
• First release with this error: 1.118.0-insider (46e64ce, Apr 23 06:57)
• Commit range: 40db3373...46e64ce9
• Users on target build: 367

Deminified Stack Summary

The listener leak originates from webviewElement.ts:loadResource (line 794/804). The subscription chain is:

1. event.ts → leak monitoring fires
2. webviewElement.ts → loadResource subscribes to an event

This is a popular leak — no single subscriber dominates; the webview element's event emitter has accumulated too many listeners across sessions.

Generated by errors-regression-scan

Generated by errors-regression-scan · ● 10.1M · ◷

[vs-code-engineering]

🔍 Error Investigation

Summary

PR #311844 ("Use proper streams for loading webview resources") introduced a ReadableStream-based path for streaming webview resources. Inside the start callback of each ReadableStream, a listener is added to token.onCancellationRequested (line 804 of webviewElement.ts). Since all resource loads share a single _resourceLoadingCts token per WebviewElement instance, loading many resources concurrently (e.g., a markdown preview with many images or videos) causes the listener count on the token's emitter to exceed the leak detection threshold, triggering this "popular" leak warning. The error affects all platforms (Mac, Windows, Linux) and has impacted ~1,247 users across 1,793 hits.

Culprit Commit

Field	Value
Commit	b980ef34
Author	@mjbvz
PR	#311844
Message	Use proper streams for loading webview resources
Why	Introduced a new ReadableStream code path where each resource load subscribes to token.onCancellationRequested(close) on a shared CancellationTokenSource. Concurrent resource loads accumulate listeners on the same emitter, exceeding the leak detection threshold.

Code Flow

sequenceDiagram
    participant WV as Webview iframe
    participant WE as WebviewElement
    participant RS as ReadableStream (per resource)
    participant CT as CancellationToken (_resourceLoadingCts)
    participant EM as Emitter (leak monitor)

    WV->>WE: load-resource (×N concurrent)
    loop For each resource
        WE->>WE: loadResource(id, uri, opts, token)
        WE->>RS: new ReadableStream({ start })
        RS->>CT: token.onCancellationRequested(close)
        Note over CT: Listener count += 1
    end
    Note over EM: ⚠️ Root cause:<br/>Listener count exceeds threshold<br/>(N concurrent loads on same token)
    EM-->>EM: 💥 ListenerLeakError thrown:<br/>"potential listener LEAK detected, popular"

Loading

Affected Files

File	Role
src/vs/workbench/contrib/webview/browser/webviewElement.ts	Crash site & root cause — loadResource() (line 794) creates ReadableStream whose start callback subscribes to shared cancellation token (line 804)
src/vs/base/common/event.ts	Leak detection infrastructure — LeakageMonitor fires ListenerLeakError when threshold exceeded (line 1068, 1272)
src/vs/base/common/cancellation.ts	CancellationTokenSource creates an Emitter<void> without a custom leak threshold (line 84)

Repro Steps

1. Open VS Code Insiders (1.118.0+)
2. Create a Markdown file that references many local resources (e.g., 30+ images or videos)
3. Open the Markdown Preview
4. Observe the "potential listener LEAK detected, popular" error in telemetry/developer tools — the shared _resourceLoadingCts.token accumulates one listener per concurrent resource load

Fix Proposal

Fix location: src/vs/workbench/contrib/webview/browser/webviewElement.ts → loadResource()

Approach: The core issue is that all concurrent resource loads share a single _resourceLoadingCts token and each adds a listener via token.onCancellationRequested(close) inside the ReadableStream.start callback. Two approaches:

1. Per-resource linked token (preferred): Create a child CancellationTokenSource linked to _resourceLoadingCts for each loadResource call. This isolates each resource's listener to its own token emitter. Dispose the child token when the stream completes or errors.

2. Raise the leak threshold: Create the _resourceLoadingCts emitter with a higher leakWarningThreshold to tolerate many concurrent resource loads. This is simpler but masks genuine leaks.

Recommended Owner

@mjbvz — culprit commit author (PR #311844) and webview area owner

Generated by errors-investigate · ● 8.4M · ◷

[mjbvz]

Likely not an actual leak since we dispose these but lets try to fix this