as you can see, there's this random titlebar button that doesn't seem to be attached to anything, and clicking it simply moves the button to the other screen, not accomplishing much. It seems to have "Always On Top" which is somewhat annoying. I like the buttons so I hope I don't have to disable the feature entirely. Does anyone have an idea what the issue may be?

Guessing, this looks like there might be an "invisible" window that has everything blanked out, just not this button as it cannot know that this button is there. Or a button "shining thru" from a window beneath? Some kind of tool or malware? Maybe already one of your applications in the tray. Otherwise, try cycling thru with ALT+TAB and starting Windows without any startup applications.

I thought it may have been BitDefender's UI but that doesn't appear to be the case. Hmm, it'd be nice if DF would list what windows/applications it's slapping a titlebar button onto so I can "blacklist" that application. Oh well, it seems to come and go at random, honestly it's not that annoying as I can just move it to the other monitor where it's not blocking anything of note.

Hey, thanks for the reply. Next time it happens I'll be sure to do that; like I said it comes and goes, but I'll return with my findings when I get them.

Well what do you know, it is bitdefender's UI window. The program is called gziface.exe; it seems to have to do with resolution changes (bitdefender's ui is known for not playing nice when the resolution changes). When a game loads up in non-native resolution, bitdefender seems to get a bit buggy. That application seems to only affect that little UI window in the system tray, which you can simply open on the second monitor, so blacklisting that, I doubt that would be an issue.

C:\Program Files\Bitdefender\Antivirus Free Edition\gziface.exe

edit: in fact here's the screenshot you wanted

I have another app to request being added to the blacklist of windows that should not show titlebar buttons.
Conexant Flow is the background audio application built into a number of HP Laptops.

I just found the Compatibility Section of the DisplayFusion settings where I can manually tell DF to not place titlebar buttons on this app. Good deal!