I have what appears to be a window which is always created in my desktop environment:
See that white bar in the upper left corner? It seems to be a 5x1 pixel window. I can drag it around. Right click does nothing. It is always present and on all desktops.
In the screenshots I am using i3 window manager, but the window is also present when I use x fce4 (via Xubuntu, the distro installed on this system).
Is there a way to determine what is creating that window and or kill it?
Found the culprit thanks to answers. It was