I had HOMM 2 running just fine on Fedora 21 with Xfce until things went wrong and I had to reinstall with Fedora 25. I have /home on its own partition, so that all of my personal stuff, including my DOSBOX games, were safe. Now, the game runs OK except for one thing: xscreensaver, the classic Linux screensaver program, doesn't recognize mouse movements or keystrokes inside the game's full screen window. Once it activates, it goes away as soon as I do anything, but it leaves the game in windowed mode for about a minute, then suddenly restores full screen until the next time. I'd really like to get this working properly, as the way it's working right now is very inconvenient. The only change I've made is to comment out a line specifying the resolution because it's no longer relevant.

That's odd. I have no problems with xscreensaver and DOSBox in fullscreen (Debian stretch). Are you running native DOSBox? Is the DOSBox you are running still from Fedora 21? If so, try installing Fedora 25 packaged one.

The executable is /usr/bin/dosbox, so it can't be part of the old installation especially as the old version didn't have this issue.

I found it: it turns out that I didn't have Wine installed, and once I'd done that, xscreensaver stopped being an issue.