As it turns out GTK3 greps the F10 and displays the “right-click” desktop menu.

To realy disable the F10 key-shortcut you have to add a new gtk3 ccs file.

~$ cat .config/gtk-3.0/gtk.css
@binding-set NoKeyboardNavigation {
unbind “<shift>F10”
}

* {
gtk-key-bindings: NoKeyboardNavigation
}