alright, no i find the way to simulate it and also work-around it:
steps to reproduce:
drag the browser tab anywhere on screen to create a “extra browser window”
behaviour:
browser is floating on the screen as a top layer but without frame and buttons to close it, because
workaround for the issue:
on mac i use shortcut to switch between windows of the same app cmd+¨ so I can see the browser frame which is behind the main window and then i can close it