someone suggested a workaround in my bug report thread that on windows if you can alt tab try windows key and tab to open a new desktop, switch to that and open task manager and then use it to close the executable. this in windows 10.
otherwise have task manager set to always be on top was another suggestion. i have not had time to play civ vi lately so i havent had chance to try either but i am pretty sure opening another desktop will work, i have done that with skyrim before. i just forgot the key shortcut for it.