How do you normally get to fullscreen..?

Anyway, RTFM.

FULLSCREEN: How do I change to fullscreen? Press alt-enter. Alternatively: Edit the configuration file of DOSBox and cha […]
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FULLSCREEN: How do I change to fullscreen?
Press alt-enter. Alternatively: Edit the configuration file of DOSBox and
change the option fullscreen=false to fullscreen=true. If fullscreen looks
wrong in your opinion: Play with the options: fullresolution, output and
aspect in the configuration file of DOSBox. To get back from fullscreen
mode: Press alt-enter again.

My special config for the program already has fullscreen=true. And, I don't want to get back from fullscreen, I want to get back to it after it comes back from the screensaver. (I'd really like to tell xscreensaver not to activate when DOSBox is running, but that's a different issue, not appropriate for here.) And, btw, once I'm in a windowed mode, alt-enter does nothing.

More info: HOMM 2 comes up in fullscreen mode. At that time, alt-enter does exactly what it's supposed to do. However, once xscreensaver comes up, the program is in windowed mode and ignores alt-enter.

So if you press alt-enter after HOMM 2 comes up in fullscreen mode, and then press alt-enter without having xscreensaver start, does DOSBox go back to fullscreen mode? Or if you press alt-enter to go into windowed mode, and then let xscreensaver start, does alt-enter still work?

I would suspect this is some kind of strange compatibility issue with SDL and xscreensaver. The ideal way to test would be with some other SDL 1.x application that can be toggled to go fullscreen. (I think ScummVM might work.)

If I use alt-enter to go to windowed mode, I can use it to get back to fullscreen mode. Once the screensaver kicks in and I tap a key to come back, I'm in windowed mode until I exit the program. I've never tried going into windowed mode before the screensaver comes up because I don't ever want HOMM 2 running in anything other than fullscreen mode.

Don't know if this anything related to the problem you're having, but in dosbox I used to have the problem that every so often when I was in full screen it would switch to a windowed mode and I couldn't switch back.
Never figured out what was the problem, but I suspected something to do with SDL, but a screensaver issue might have fit in there to.
The solution I found was to minimize it using the panel, then reactivate it, and I could control things again. Problem is I was doing it on an older distro, about gnome 2.x, and I imagine the Fedora 25 is using gnome 3.x or higher so most of that functionality, and probably the panel itself has been removed by now. And that assumes the problem is anything similar to do what you're facing.

Yes, F 25 uses Gnome 3.x by default, which is why I use Xfce. I'm starting it from a desktop icon that invokes a shell script (Linux equivalent of a batch file) that runs dosbox with a custom .conf file that sets things up and runs the game. I ended up by adding this line just before starting dosbox:

xscreensaver-command -exit

This tells the screensaver to exit completely. Then, after dosbox exits, the script runs this:

xscreensaver &

This starts the screensaver in daemon mode in the background. It's not elegant, and a bit of a kludge (I'd prefer a way to pause the screensaver instead of exiting completely.) but it does work. And, in my case I know it's the screensaver because it comes up just like it would if I walked away from the computer too long.

Make sure the mouse is captured.

Click inside the DOSBox window or press Ctrl+F10.

The mouse has to be captured to use the alt key at all when you're running in Linux, or at least that's been my experience.

Been there, tried that. Didn't work.

A few suggestions:
- (I know this is a bad idea) Disable screen saver?
- After screen saver is shown, and you get back to DOSBox, maybe try pressing on Alt-Enter *twice*? (On the first time, it may behave like fullscreen is toggled off, even though it already is; Only on the second time it might toggle fullscreen on again.)

Also, is this clean install of Fedora 25 the first time you're using it? Maybe you have Wayland running instead of X11, which may lead to some issues atm.