https://bugs.winehq.org/show_bug.cgi?id=37937
--- Comment #4 from Stefan Dösinger stefan@codeweavers.com --- I think running the game in a virtual desktop at the size of your real screen will do what you need. If the desktop size matches the screen resolution Wine will disable the desktop borders, essentially giving you a fullscreen game that is impervious to focus loss.
If you need a resolution switch at the same time things get more difficult though.
@Sebastian: I think Forest's use case is a bit non-standard, though I can understand the reasoning behind it. In the past I have used multiple KDE desktops (not wine virtual desktop) as a workaround for Wine's lack of fullscreen focus loss handling, Forest has just taken it one step further.