What happens if the UI process/thread thinks that a point only has non-canceling event listeners, but when the event reaches the main thread, a regular event listener that calls preventDefault() has been added? It's too late to actually prevent the scroll, but it'll look to the event listener like it can prevent it.

Perhaps create non-cancelable events when the UI process/thread actually isn't waiting for the result? Though this would make the presence of other no-op event handlers observable, ugh.