Comment 3 for bug 1649136

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.