Allowing event propgation and default behaviour on mouse scrolling

This change stops multiple selleckts from preventing the user scrolling the element behind, on when the multiple selleckt element is closed.

Waiting for:

 - [ ] QA
This `{` needs to be on the live above

I'd also invert this check so that we return early (could just be a `return;`, doesn't have to `return true;`) if the menu isn't open