[MU4 Issue] Multiple monitors - Panel can't be on bottom edge of mac bar and docking area is glitched.

[jessjwilliamson]

Describe the bug 2 issues to describe here:

1. A panel can't be moved to the very top of the screen on 2nd monitor. There are a few pixels of space preventing it to move to the top.
2. When I place a panel on 2nd monitor, 1st monitor thinks i'm still 'docking' hence the blue area being highlighted.

https://user-images.githubusercontent.com/71656189/141492305-3071741a-f8af-4177-8ec2-4c1bf10a6d62.mp4

Desktop (please complete the following information):

• OS: Mac OS

[zacjansheski]

I am unable to reproduce the second point

The first point (space between panel and top menu), seems to be an issue on a single monitor as well

[cbjeukendrup]

The few pixels space around undocked panels is for the drop shadow, which we render ourselves instead of letting the OS render it around the window. But in order to be able to render custom drop shadow, we need to make the window a bit bigger than its content, because we can't paint outside the window.

This also causes https://github.com/musescore/MuseScore/issues/10020.