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

docs/user.md

@@ -598,7 +598,12 @@ keys** checkbox groups for keypad Insert, Insert, Caps Lock, and Super; Compose
 is also available for scripts. Uncheck a key in one group before assigning it
 to the other. A physical key cannot serve both roles, including through an
 alias such as `KEY_META` and `KEY_RIGHTMETA`. At least one Fenrir key must remain
-selected. Changes take effect on the next launch.
+selected. After **OK** saves the changes, modifier assignments apply when the
+menu closes and all held keys are released. Cancel leaves them unchanged.
+If X11 cannot acquire the new keys, Fenrir reports the failure and retains the
+previous active assignments. The saved choices remain in the file; reopen the
+menu to edit or retry them. Desktop/laptop layout changes still require a new
+launch.
 
 The Fenrir-key checkboxes save `keyboard.fenrir_keys`, which overrides the
 layout's defaults. When that setting is omitted, the layout supplies defaults