For a short while now, MC has been behaving erratically for me:
For example, I'd tell it to play an Album, and, sometime between approximately 00:50 and 02:00 minutes, the Playing Now would change itself to playing 100 random tracks from my library (which it also would not play in full, but rather it would start playing the first track, then after a while it would switch to the second one, and so on. I'm positive that I did nothing to select the '100 random songs' smartlist, and also, the Preview Mode is set to play the entire song.
Another example is that I'd tell MC to play something, and it does that, but I then switch to another application, do things there, and suddenly MC enters Theater View and steals focus. When I then go back to doing what I was actually doing, this process will repeat as long as MC is open. In fact, that even happened while writing this post. I managed to capture a log of this, which I will attach.
I am fairly certain (read: I did not pay attention to this when it first happened, because I was tired) that this started occurring only after I updated to 21.0.19, but I think I remember (again, I was tired) that the playlist changing stuff kept occurring even after I had downgraded to 21.0.17. Because of that, I went back to .19.
From reading the log, it seems to me that MC thinks it's getting some hotkey event, but I'm uncertain as to which one, and where it's supposed to be getting it from. Just for fun, I tried the hotkey for Theater Mode (CTRL+4) while the browser had focus, and nothing happened (except that the browser switched to the fourth tab). On the other hand, twice now I've done nothing (not even typed something), and MC still entered Theater Mode by itself.
One thing to note for both of these examples is that the mouse pointer switched from its normal arrow symbol to the 'busy' sign whenever the issue occurred. Maybe it turns out to be irrelevant, but I don't know...
If you need more information, or if I can help in another way, please tell me.
Another crazy thought that just occurred to me: Is it possible that MC somehow recognizes my Bluetooth mouse as a remote? I mean, sure, it shouldn't, and I can also use the mouse fine inside MC, but could this be related? (Although, I've had this mouse for a few weeks already, without problems.)
Edit: So I was curious, and left a keylogger run in the background, and waited. Sadly, as I had expected, it caught no keyboard/mouse events at all before MC entered Theater Mode.