With KRunner being a "dock" / panel, it won't go ontop of fullscreen windows. On X11 I'm able to Alt+Space even while watching a fullscreen movie.
I cannot reproduce. If I have a fullscreen window and open KRunner, the fullscreen window gets deactivated and is stacked behind the panel and krunner. With which application do you have this problem? I just tested with Firefox.
This seems to be related to the window which goes fullscreen. E.g. in kwrite if I set to fullscreen Krunner goes on top. But for firefox it does not.
*** This bug has been marked as a duplicate of bug 375759 ***