fix: don't let a listener exception wedge EventEmitter

[Joaquín Ruales (jruales)]

Authored by GitHub Copilot on behalf of Joaquín Ruales (@jruales).

Fixes #2401

Problem

EventEmitter.fire leaves _deliveryQueue populated when a listener throws:

if (!dispatch) return;
for (let index = 0; index < this._deliveryQueue.length; index++) {
  const { data, event } = this._deliveryQueue[index];
  data.listener.call(data.thisArg, event);   // throws
}
this._deliveryQueue = undefined;             // never reached

Because the queue is never cleared, every later fire computes dispatch === false, enqueues, and returns without dispatching. One listener exception permanently stops all delivery on that emitter.

Connection reads its transport through this._transport.onMessage(...), so this silently kills the CDP message pump for the whole connection. The debuggee is then never sent Runtime.runIfWaitingForDebugger and stays paused at startup — the window paints but accepts no input.

The exception comes from Connection._onMessage, which ignores messages for disposed sessions but throws for unknown ones. Inspector.workerScriptLoaded (a Blink event fired when a web worker finishes evaluating) is never handled by js-debug and can arrive before the worker's session is registered, since targetCreated is queued through enqueueLifecycleFn:

Error: Unknown session id: B9878282765DBAAB165E4A5851627507 while processing: Inspector.workerScriptLoaded
    at ut._onMessage (.../src/extension.js:58:8166)
    at U.fire        (.../src/extension.js:39:11654)

Changes

• src/common/events.ts — restore _deliveryQueue in a finally block so one bad listener can't wedge the emitter. This is the load-bearing fix and is independent of the CDP specifics.
• src/cdp/connection.ts — treat unknown sessions the way disposed sessions are already treated (warn and ignore) instead of throwing.
• src/common/events.test.ts — regression test.

Validation

• New test fails without the events.ts change (expected [ 'before' ] to deeply equal [ 'before', 'after' ]) and passes with it.
• npm run test:unit — 268 passing. tsc --noEmit and gulp lint clean.
• End-to-end A/B: this extension built both ways, loaded into a real VS Code Insiders window via --extensionDevelopmentPath; same machine, repository (microsoft/vscode, Electron 42.8.1), isolated profile and launch compound, only this diff varying.

Build	Outcome
Unpatched	Workbench freezes. Reproduced.
Patched	Works every time, across repeated runs.

Happy to split the connection.ts change into a separate PR if you'd prefer to evaluate the two independently.