Brief flash of buttons when closing

This could be specific to my layout but I'm seeing a brief flash of the buttons on closing. It seems to occur at the point where @.is-closing@ is removed from the dialog, though it's very hard to tell in the dev tools.

On a hunch, I added `&:not([open])` to the classes that set the buttons to zero opacity around https://github.com/deoostfrees/Parvus/blob/1fd9a326c576b027d32a441f160920cf2ed05447/src/scss/parvus.scss#L286-L287 of the scss and it seems to have helped.

```
&:not([open]),
&--is-opening,
&--is-closing {
```

Could I potentially be causing myself problems with this?