After remotely closing some windows with DISPLAY=:0 wmctrl -ic 0x058001b2 I looked into ~/.xsession-errors and discovered these messages: Window manager warning: Receiving a NET_CLOSE_WINDOW message for 0x58001b2 (Indoor pos) without a timestamp! This means some buggy (outdated) application is on the loose! Window manager warning: Tried to ping a window with CurrentTime! Not allowed. I use Mate with Marco on Lightdm, all updated to current testing distribution