There is only "leave fullscreen" available. AFAIR it was possible to configure this in the past.
Created attachment 113801 [details] Screenshot shortcut editor I can't confirm that with 18.04 or master. Both versions offer a "toggle" and "leave" shortcut. Are the default shortcuts F11 / Ctrl+Shift+F working for you? Could you check with a clean config (remove/rename the files ~/.config/gwenviewrc and ~/.local/share/kxmlgui5/gwenview/gwenviewui.rc)?
Thx. Was false expectation (and I didn't use the search bar). It appears in the "VIEW" menu, so I expected it to also appear in that section of the shortcut editor, where you also can find "Leave Fullscreen". But it seems "toggle fullscreen" is a toplevel-shortcut.