Fix listener removal when listener is not found

[kiran1929]

The current implementation directly calls:

this.listeners.splice(this.listeners.indexOf(listener), 1);

If the listener is not present, indexOf() returns -1,
causing splice(-1, 1) to silently remove the last listener
instead of doing nothing — corrupting the listener array.

=> Fix
Added an index guard before splicing:

const idx = this.listeners.indexOf(listener);
if (idx >= 0) {
this.listeners.splice(idx, 1);
}

=> Testing

• Added a unit test in errors.test.ts covering the
  case where an unregistered listener is removed
• npm run transpile-client
• ./scripts/test.sh --runGlob '**/errors.test.js'

[kiran1929]

@microsoft-github-policy-service agree

[Copilot]

Pull request overview

Fixes a correctness issue in ErrorHandler where removing a listener that isn’t registered would previously remove the last listener due to splice(-1, 1).

Changes:

• Guard listener removal by checking indexOf result before calling splice.
• Add a unit test intended to cover removing an unregistered listener (currently not exercising the behavior).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/vs/base/common/errors.ts	Prevents accidental removal of the last listener when attempting to remove a non-existent listener.
src/vs/base/test/common/errors.test.ts	Adds a test placeholder for the regression scenario (needs to be updated to actually validate behavior).