When I move a window to the top left corner, it moves to a different desktop. I suppose that is some mouse-gesture feature, but I can't find that in the system settings.

<= 4.4:
System Settings -> Desktop -> Screen Edges -> Switch desktop on edge.

Ah, so that is what that means. But it is set to 'disabled'.

Hm right, you said in another topic that you use Openbox as window manager, didn't you? In that case, it's an Openbox feature, so you should find an option to disable it in the Openbox settings.

Ah, I found Settings > Openbox Configuration Manager, where I found "switch desktops when moving a window past the screen edge". Disabled that. Problem solved. Thanks!