Reviving this old thing to complain that the issue is now even stranger in the Big Sur Beta. Originally the Media Keys controlled iTunes and were great. They worked every time. Then the issue this thread addresses came up: Media Keys input was just handed off to whichever app was in the foreground at any time. That was silly because the foreground app would already have its on-screen controls (and likely the spacebar) available to it for play/pause. Now in Big Sur, the behavior is even worse.  If I open Music and play some music, then open Safari -> YouTube, play a video, and hit the Play/Pause key, the video will pause. BUT, if I then bring Music back to the foreground and hit the Play/Pause key again, control REMAINS with the YouTube video. It's so frustrating.  There have been multiple 3rd party apps which have come and gone dealing with just this one obnoxious behavior. Now, after years of the "most recent foreground" behavior and no word from Apple, it has been updated once again and makes less sense than ever. I wrote my own app which intercepts those key presses and forces them to Music (more or less a rewrite of a small section of the MadMediaKeyForwarder project here: https://github.com/milgra/macmediakeyforwarder), but I would much prefer just a toggle or some setting in macOS which makes these keys anything close to useful again. Even just an explanation as to the reasoning or expected behavior of these keys would be great at this point. Ideally they could just be priority-mapped to certain applications or something. 

1 for this, haven't been able to figure out a cause. Previews actually seem to work most of the time, but will randomly break and I'll see this error.

Same issue here. I'll give VMWare and see what happens.

No, according to this page: https://www.apple.com/macos/big-sur-preview/ Also it's 11.0, not 10.16.