The animated demo works, then doesn't, then does, then doesn't, ...

The animated demo in `demo/index.html` works as expected the first time, but not the second: the modal dialog appears, but it closes without animation (via inspector I see both the modal and the overlay end up with `display:none`).

If I click the button a third time, it works as expected again, but the fourth has the problem above again.

Tested with Chrome 78 and Firefox 70 on Windows 10.