OS Ubuntu 16.04
QGIS version 2.18.16
I have a problem where if I try to open any windows that spawns from "Rule-based" symbology in the style menu. Trying to change the rules by double clicking on them, or to change the symbology results in the application getting locked, I am assuming because the window that is spawned is somehow off-screen. I have no idea how this even happened because I am only using one screen, and it was working earlier, but all of a sudden it does not.
The only way to get control back is to manually kill QGIS via the terminal and restart it.
The only place this seems to happen is when I try to spawn the windows from the "Rule-based" symbology menu, other symbology menus (categorized, single symbol etc.) do not cause a problem (spawned window appears on-screen)
This solution did not help because it is for a toolbar and not a window.