Works fine, except our keyboard emulation and Bus/PS/2 mouse emulation does an extraneous check for fullscreen that will not work properly if the UI is on while fullscreen. It should be accounted for. Everything else seems fine.

OK this was adjusted to account for those fullscreen checks in those hardware files. The variable was also converted to a C int to match the way the "fullscreen" flag is implemented.

Undefined variables in the SDL builds:

/usr/bin/ld: src/device/CMakeFiles/dev.dir/keyboard.c.o: in function `keyboard_input':
/home/runner/work/86Box/86Box/src/device/keyboard.c:375: undefined reference to `fullscreen_ui_visible'
/usr/bin/ld: /home/runner/work/86Box/86Box/src/device/keyboard.c:375: undefined reference to `fullscreen_ui_visible'
/usr/bin/ld: src/device/CMakeFiles/dev.dir/mouse_bus.c.o: in function `bm_update_data':
/home/runner/work/86Box/86Box/src/device/mouse_bus.c:546: undefined reference to `fullscreen_ui_visible'
/usr/bin/ld: /home/runner/work/86Box/86Box/src/device/mouse_bus.c:546: undefined reference to `fullscreen_ui_visible'
/usr/bin/ld: src/device/CMakeFiles/dev.dir/mouse_bus.c.o: in function `bm_poll':
/home/runner/work/86Box/86Box/src/device/mouse_bus.c:484: undefined reference to `fullscreen_ui_visible'
/usr/bin/ld: src/device/CMakeFiles/dev.dir/mouse_bus.c.o:/home/runner/work/86Box/86Box/src/device/mouse_bus.c:484: more undefined references to `fullscreen_ui_visible' follow

Thanks, I hadn't realized that was a thing. I thought it was all QT. I moved it to the root 86box.c.

This PR adds the shortcut key of CTRL+ALT+PGDN to toggle the UI while in full screen. When it's toggled, the mouse is released (if it was captured) and the shortcut then hides the UI again, restoring the mouse to its previous state.

Please, can you add another option to the menu? Automatically capture mouse in fullscreen mode when the machine is started with the -F parameter (when the machine is started immediately in fullscreen mode).