Created attachment 3672 
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I have two panels - one on the bottom with Length = 100% and one small for plugins, etc...
I used to have the second plugin at the top of the screen. The panel was like "on the top of" other windows and windows could be moved under the panel.
Since last update (to 4.8.3) when I move the panel to the top of the screen, no other window can be moved under the panel - which makes whole stripe of the screen unused. See the picture attached - i can not move the ff window higher because of the panel. And it was possible in previous versions.

Note: it is possible to do it when using "hiding mode" - panel is hidden and when cursor move over it, it draws itself over the windows. but i want to see the panel all the time (and it was possible)