I know we've had a policy to avoid window manager specific workarounds, I think we could reconsider it. The fix could be unguarded and applied with every window manager, and could very well work, but I think it is ugly enough to justify checking for KWin specifically.
The problem is that as soon as a window configure request is sent to a maximized window, KWin internal state gets bogus and it loses track of the window maximized state. I've described the KWin source details on https://bugs.kde.org/show_bug.cgi?id=496966 and opened https://invent.kde.org/plasma/kwin/-/merge_requests/6854 as a possible fix for it, but the time it will take to land could justify working around it in Wine.
The workaround is to avoid sending configure requests to maximized windows, which sounds sensible and we already avoid resizing maximized windows, but, moving a maximized window to a different monitor *requires* sending a configure request. KWin bug makes no difference to requests with only position changes, and they trigger it all the same. So, the only solution is to temporarily remove the maximized state bits before sending the configure request, putting them back afterwards. This is quite straightforward to do with the new state tracker, but it could very well trigger other problems with other window managers.
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=57465
-- v3: winex11: Workaround KWin bug with maximized windows. winex11: Keep track of the window manager name. winex11: Keep track of the _NET_SUPPORTING_WM_CHECK window. winex11: Listen to root window _NET_SUPPORTED property changes. winex11: Move the _NET_SUPPORTED information to the thread data.