Say i have some (react) code like:

someCondition 
  ? <MyComponent onSomeEvent={handleSomeEvent} />
  : <MyComponent />

Then the event handler doesn’t get removed when someCondition is false.

The current code for syncing props, only considers new props when adding/removing event handlers, and doesn’t remove handlers that existed in previous props but not in new props.

This PR modifies the attachProps util to call removeEventListener on any listener which is not part of the latest props.

I had a little trouble writing a good test (read: not testing implementation details), since I think testing-library does not work well with web components. So I just followed the approach in other tests (inspecting __events) instead.

I also attempted to fix the build, as the file:// urls for the svelte output target were causing issues.