[Unhandled Error] ListError [AgentSessionsView] Invalid index 0

[vs-code-engineering]

Error Message

ListError [AgentSessionsView] Invalid index 0

Stack Trace

    at oui.getRelativeTop (./src/vs/base/browser/ui/list/listWidget.ts:2004:9)
    at iEe.getRelativeTop (./src/vs/base/browser/ui/tree/abstractTree.ts:3110:0)
    at Ey.getRelativeTop (./src/vs/base/browser/ui/tree/asyncDataTree.ts:1045:0)
    at XW.reveal (./src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts:836:0)
    at (anonymous) (./src/vs/workbench/contrib/chat/browser/widgetHosts/viewPane/chatViewPane.ts:973:0)

Raw Stack Trace (minified)

Error: ListError [AgentSessionsView] Invalid index 0     at oui.getRelativeTop (out/vs/sessions/sessions.desktop.main.js:728:13602)     at iEe.getRelativeTop (out/vs/sessions/sessions.desktop.main.js:861:2941)     at Ey.getRelativeTop (out/vs/sessions/sessions.desktop.main.js:861:37132)     at XW.reveal (out/vs/sessions/sessions.desktop.main.js:3220:72945)     at out/vs/sessions/sessions.desktop.main.js:3220:90280

Details

Property	Value
Bucket ID	c090838c-4fb8-75d7-43bf-104faeab8217
Version	1.116.0-insider
Commit	1dfb3b9d
Last Seen	Mon Apr 13 2026 22:11:49 GMT+0000 (Coordinated Universal Time)
Platforms	Windows, Mac
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 (47add6df)
• First release with this error: 1.116.0-insider (1dfb3b9d)
• Commit range: 47add6df...1dfb3b9d

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

[vs-code-engineering]

🔍 Error Investigation

Summary

The error ListError [AgentSessionsView] Invalid index 0 is thrown when AgentSessionsControl.reveal() calls getRelativeTop() on the sessions list tree while the tree model and list widget are out of sync. The tree model reports a node exists at index 0 but the list widget has 0 rendered items. This is a race condition triggered during orientation transitions (switching to side-by-side view) in the Chat View Pane.

Culprit Commit

No narrowed commit range was provided by the regression scan. Based on telemetry bucket history, the error first appeared at 3d70aab9 (1.115.0-insider, Apr 3) with 1 hit, then increased across 1.116.0-insider builds (up to 12 hits, 8 users). The reveal() method with the getRelativeTop pattern was originally introduced in PR #290233 (merged Jan 25), but the error only started appearing later — likely exposed by subsequent changes to session list layout, grouping, or visibility logic.

The root cause is a missing guard in the reveal() method itself, rather than a specific commit introducing a regression.

Code Flow

sequenceDiagram
    participant Layout as ChatViewPane.layoutSessionsControl
    participant Control as AgentSessionsControl
    participant AsyncTree as AsyncDataTree
    participant AbstractTree as AbstractTree
    participant ListView as ListWidget

    Layout->>Control: update() [via Throttler]
    Control->>AsyncTree: updateChildren()
    Note over AsyncTree: Tree model updated,<br/>nodes map populated
    AsyncTree-->>Control: Promise resolves
    Layout->>Control: reveal(sessionResource)
    Control->>Control: hasNode(session) → true ✅
    Control->>AsyncTree: getRelativeTop(session)
    AsyncTree->>AbstractTree: getRelativeTop(dataNode)
    AbstractTree->>AbstractTree: model.getListIndex() → 0
    AbstractTree->>ListView: getRelativeTop(index=0)
    Note over ListView: 💥 this.length === 0<br/>0 >= 0 → throws ListError

Loading

Affected Files

File	Role
src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts	Root cause — reveal() method lacks guard before getRelativeTop() call (line 836)
src/vs/workbench/contrib/chat/browser/widgetHosts/viewPane/chatViewPane.ts	Trigger — orientation switch to side-by-side calls reveal() after async update (line 987–992)
src/vs/base/browser/ui/list/listWidget.ts	Crash site — getRelativeTop() throws ListError when index ≥ length (line 2003–2004)
src/vs/base/browser/ui/tree/abstractTree.ts	Data flow — getListIndex() returns valid index from tree model, passes to list widget (line 3103–3110)
src/vs/base/browser/ui/tree/asyncDataTree.ts	Data flow — hasNode() checks nodes Map (different structure than list widget length)

Repro Steps

1. Open VS Code Insiders (Agents product) with the Chat View Panel visible
2. Have at least one agent session in the sessions list
3. Resize the Chat View pane width rapidly across the SESSIONS_SIDEBAR_VIEW_MIN_WIDTH threshold — this forces repeated orientation switches between stacked ↔ side-by-side
4. The race window exists between update() completing (tree model updated) and the list widget fully rendering its items
5. Alternatively, this can trigger during initial layout when the chat view pane is first opened in a width that selects side-by-side orientation

This is a race condition and is non-deterministic. Rapid resizing or slow data source responses increase the likelihood.

Fix Proposal

Fix location: src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts → reveal()

Approach: Wrap the getRelativeTop() call in a try-catch to gracefully handle the case where the tree model and list widget are out of sync. If getRelativeTop() throws, fall through to reveal(session, 0.5) unconditionally (which is the desired behavior when the element isn't visible anyway):

reveal(sessionResource: URI): boolean {
    // ... existing checks ...

    try {
        if (this.sessionsList.getRelativeTop(session) === null) {
            this.sessionsList.reveal(session, 0.5);
        }
    } catch {
        // Tree model and list widget can be temporarily out of sync
        // during orientation transitions — fall through to focus/select
    }

    this.sessionsList.setFocus([session]);
    this.sessionsList.setSelection([session]);
    return true;
}

Alternatively, a more targeted fix would check the list widget's length before calling getRelativeTop, but the try-catch is more resilient against other edge cases in the tree widget lifecycle.

Recommended Owner

@bpasero — primary author of the chat sessions integration code in chatViewPane.ts and the agent sessions layout logic (PRs #289325, #305555, #284396). The reveal() call site in the orientation switch code falls within his ownership area.

Generated by errors-investigate · ● 14.5M · ◷