Snapshot the listeners emit() walks

[enyo]

Fixes #2367 (the emitter part — see the note at the bottom about the bonus findings).

The bug

emit iterates the live array:

for (let callback of callbacks) {
  callback.apply(this, args);
}

and off splices in place:

callbacks.splice(i, 1);

for…of over an array walks by index, so removing the entry at the current index shifts everything after it down one and the loop's next step lands past the neighbour. With [a, b, c] and a removing itself, b never runs.

A listener that unregisters itself is not exotic — it is how you write a one-shot listener, and it is what teardown code does from inside a complete or queuecomplete handler.

The fix

Iterate a copy, which is what Node's EventEmitter does.

Behaviour change worth flagging

The snapshot cuts both ways: a listener registered from inside another listener used to run in that same emit (the live array grew under the loop) and now first runs on the next one. That is the standard semantics and there is no way to keep the removal case correct without it, but it is a real change, so it is called out in the changeset and covered by a test that pins the new behaviour.

Tests

Three, in test/unit-tests/emitter.js. Against main:

• a self-removing listener: expected [ 'first', 'third' ] to deeply equal [ 'first', 'second', 'third' ] — the skip, exactly as reported
• a listener added mid-emit: expected [ 'first', 'added' ] to deeply equal [ 'first' ] — the behaviour change above
• off(event) removing everything mid-emit already passed (it deletes the key rather than mutating the array); it is there as a guard.

The bonus findings in #2367

Both are real — destroy()'s splice(indexOf(this), 1) evicting a live instance on -1, and init() comparing tagName to lower-case "form". They have nothing to do with the emitter, so they are not in this PR. I have them prepared on a branch; the enctype one is a dormant behaviour change (a natively-submitted form would start encoding as multipart/form-data), so it seemed worth your call rather than mine.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	79.42%	880 / 1108
🔵	Statements	79.77%	927 / 1162
🔵	Functions	92.89%	196 / 211
🔵	Branches	76.46%	510 / 667

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
packages/dropzone/src/emitter.ts	100%	100%	100%	100%

Generated in workflow #141 for commit a1a67df by the Vitest Coverage Report Action