Attempt to make modifier key changes take affect when the dialogue closes. This means you don't have to restart Fenrir to apply them.

README.md

@@ -220,7 +220,10 @@ The keyboard layout combo box offers Desktop and Laptop. The Keyboard tab also
 has separate Fenrir-key and script-key checkboxes for keypad Insert, Insert,
 Caps Lock, and Super, plus Compose for scripts. Uncheck a key in one group
 before selecting it in the other. At least one Fenrir key must remain selected.
-Key assignments take effect on the next launch.
+After **OK** saves the changes, modifier assignments apply when the menu closes
+and all held keys are released. If X11 cannot acquire a new key, Fenrir reports
+the failure and keeps the previous assignments active. The saved choices remain
+available for retrying or editing in the menu.
 Backend, device, keyboard layout, sound theme, and other launch-only changes
 take effect the next time Fenrir starts.