fix: guard terminal resize against disposal to prevent xterm dimensions error (fixes #316419)

[vs-code-engineering]

🔧 Error Fix

Summary

Error: Cannot read properties of undefined (reading 'dimensions') — a TypeError thrown by xterm.js's Terminal.dimensions getter when its internal RenderService._renderer.value is undefined post-disposal.

Impact: 48.6M hits across 4.9M users. Affects all platforms (Linux, Mac, Windows). Bucket 1e83a096-ed50-2ba1-2dd8-46381020bd02.

Root cause: During TerminalInstance.dispose(), the _onDisposed event fires at line 1327 before super.dispose() is called at line 1345. This means isDisposed is still false during the event chain. Listeners triggered by _onDisposed can dispose timers/schedulers whose callbacks fire configuration change events. These config changes reach setVisible() on the disposing terminal, which calls _resize() → debouncer → _resizeBothCallback → _updatePtyDimensions(). The existing isDisposed guards in the callback and _updatePtyDimensions pass (returning false), and the xterm dimensions getter throws because the renderer is in a bad state during the disposal process.

Fixes #316419
Recommended reviewer: @Tyriar

Culprit Commit

This is a long-standing issue (bucket first seen at v1.109.5, March 2026) exacerbated by the xterm.js beta.213 bump (644c7175 by @bryanchen-d). The prior fix attempt added isDisposed guards and a try/catch (644c7175), then the try/catch was removed (5e647b29) on the assumption that isDisposed guards were sufficient. They are not sufficient for the _onDisposed.fire() path where isDisposed is still false.

Code Flow

sequenceDiagram
    participant PE as _onProcessExit
    participant D as dispose()
    participant EV as _onDisposed listeners
    participant CC as Config Change
    participant SV as setVisible
    participant R as _resize
    participant DB as ResizeDebouncer
    participant UP as _updatePtyDimensions
    participant XT as xterm.dimensions getter

    PE->>D: dispose(TerminalExitReason.Process)
    Note over D: isDisposed = false (super.dispose() not yet called)
    D->>EV: _onDisposed.fire(this)
    EV->>CC: listener disposes timer, timer callback fires
    CC->>SV: config change triggers setVisible(true)
    SV->>R: _resize()
    R->>DB: resize(cols, rows, immediate)
    DB->>UP: _resizeBothCallback, _updatePtyDimensions(xterm.raw)
    UP->>XT: rawXterm.dimensions
    Note over XT: renderer undefined, TypeError

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Affected Files

File	Role
src/vs/workbench/contrib/terminal/browser/terminalInstance.ts	Terminal instance with dispose/resize logic

How the Fix Works

Chosen approach (terminalInstance.ts): Added a _isDisposing flag that is set to true at the start of dispose() (before _onDisposed.fire()) and checked in _resize() alongside isDisposed. This guards against resize operations during the entire disposal process, not just after super.dispose() completes. The fix is at the data producer level — preventing the resize call from being made when the terminal is mid-disposal, rather than catching the error at the crash site (the xterm getter).

Alternatives considered:

• Adding a try/catch around the dimensions access: wraps the crash site, hides the bug.
• Only checking isDisposed in _resize: insufficient because isDisposed is false during _onDisposed.fire().
• Moving super.dispose() before _onDisposed.fire(): breaks contribution addon cleanup.

Recommended Owner

@Tyriar — primary owner of the terminal component. @bryanchen-d previously worked on this exact bucket.

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