I have this since somedays, that i cannot switch from Widelands to another window.
I tried to find out what the trigger is but got only working...
Now i have it again, reading news on another desktop 3 and switching back to desktop 2 and can't get "behind" the game window.
Also i was not able to click into the game to activate the window, i had to use the icons on the panel.
Trying to switch with ALT+Tab gives graphic errors, the "menu" for this function (showing all open windows on this desktop to switch through) comes not up and the screen is partly flickering with the images fromthe window behind and so on...
Only solution:
Click icon on panel, leave the game.
Sounds for me like a hardware problem so some links:
https://forum.manjaro.org/t/dump-your-inxi-fxzc0-here-and-bookmark-it/8298/141?u=laza
OS:
Manjaro Xfce 64 Bit
Had this issue also now. After closing widelands i saw that every decoration of all windows are lost (borders, window titlebar). I new this situation and how to solve it: Just restart the Displaymanager (here by calling 'kwin_x11 --replace'). After restarting the displaymanager i could start widelands again without any problems.
Maybe your displaymanager has similar command to restart.