caolan When I click on Choose Tags straight from the new post form, the window with blue overlay opens, but the window disappears and only the blue overlay stays.
I can confirm that. In normal, not maximized Firefox window Choose Tags dialog opens and immediately disappears:
Choose Tags dialog does not disappear in maximized Firefox window:
Edit: Choose Tags disappears in not maximized Firefox window in all DEs - Plasma, Gnome, Mate, Budgie.