If the argument is @nil, this is equivalent to menu->clear_selection,

• unless* there is a menu_item with menu_item<-value. Backward compatibility.

  Bugs: Does not check menu<-multiple_selection, resulting in an inconsistent menu when multiple object are selected in a menu with menu<-multiple_selection: @off.

  See also

  - menu<-selection
  - menu_item-selected
  - menu->clear_selection