https://bugs.winehq.org/show_bug.cgi?id=56149
--- Comment #9 from Rémi Bernon rbernon@codeweavers.com --- I don't see the issue on GNOME, but I also don't have a taskbar.
Kwin is known to have issues with leaving its taskbar over fullscreen windows, as well as incorrect focus state, under some convoluted circumstances. This is a kwin bug (reported as https://bugs.kde.org/show_bug.cgi?id=466160) and it should be fixed there, not in Wine (otherwise they will keep not fixing bugs).
Some other WM might have bugs as well, and they should fix them as well. I think having workarounds in Wine has always been frowned upon, first because it makes Wine (and esp winex11) code more complicated, and second because working around an environment bug simply hides it and makes it less likely to be fixed.
Wine triggers a lot of WM bugs, because of our very specific request pattern, still doesn't mean we should consider it's our fault. We don't do any kind of dupe request check when making or removing fullscreen from normal window, I don't see why we should here.