There is at least one window type (the job change dialog) where you can open multiple copies that have to be closed individually. In this case, they overlap precisely and don’t darken the remainder of the screen like in the bug reported here, which is also a bit old, and about a different window type (hence my new post).
See the video at around 1:51. (Listen to the opening and closing sounds, as you cannot otherwise tell when I’m clicking.)