WebglAddon.dispose() does not release the underlying WebGL2 context #6068

Details

• Browser and browser version: Chromium (reproduced on Electron 30+/Chromium 124+ and stock Chrome); driver-dependent
• OS version: Windows 11 (also seen on macOS)
• xterm.js version: 5.5 / current master, @xterm/addon-webgl

Steps to reproduce

1. Create a Terminal, open() it, and load a WebglAddon.

2. Dispose the addon (or the terminal), then repeat: create + dispose many WebGL-backed terminals over the app's lifetime (tab/split churn, session restore, etc.).

3. Once the number of contexts created this way passes Chromium's per-document live-context cap (~16, driver/build dependent), the console logs:

   WARNING: Too many active WebGL contexts. Oldest context will be lost.
   

   and a still-live terminal loses its renderer (webglcontextlost) and renders blank / as an "X box" / garbled.

A self-contained repro is attached (repro.html): it keeps one live terminal and then creates and disposes 30 more WebGL terminals.

Root cause

WebglRenderer's dispose (registered in the constructor) tears down the render layers and removes the <canvas> from the DOM, but never releases the WebGL2 context:

https://github.com/xtermjs/xterm.js/blob/master/addons/addon-webgl/src/WebglRenderer.ts#L160-L168

this._register(toDisposable(() => {
  clearTimeout(this._contextRestorationTimeout);
  this._contextRestorationTimeout = undefined;
  for (const l of this._renderLayers) {
    l.dispose();
  }
  this._canvas.parentElement?.removeChild(this._canvas);
  removeTerminalFromCache(this._terminal);
}));

Detaching the canvas does not free the GPU-side context — it lingers until the canvas is garbage collected, which is non-deterministic and can be arbitrarily delayed. Every disposed addon leaves a "zombie" context that still counts toward Chromium's cap. When the cap is exceeded the browser force-evicts the oldest context, which may belong to a terminal that is still on screen, so a live terminal goes blank rather than the disposed one being cleaned up.

Impact

Any embedder that creates and disposes many terminals in one document hits this — VS Code-style multi-terminal UIs, terminal multiplexers, IDEs with per-tab terminals. The failure is confusing because the terminal that breaks is not the one that was disposed, and it only appears after enough churn to cross the (driver-dependent) cap.

We currently work around it downstream by reaching into private addon internals (addon._renderer._gl) after dispose() and calling WEBGL_lose_context.loseContext() ourselves — which is exactly the kind of thing that should live in the addon.

Suggested fix

Call this._gl.getExtension('WEBGL_lose_context')?.loseContext() as part of the renderer's dispose so the context is released immediately and the live count stays bounded. PR to follow.