I'm using Ubuntu 16.04 w/ Openbox 3.6.1. When I press Alt, some applications shows their menu. How I can disable this behavior, so Alt won't do anything?
Supposedly this can be disabled by changing a setting in the gtkrc or settings.ini files, for GTK 2 and 3 respectively. The option is gtk-enable-mnemonics, it has been deprecated, but the documentation states it still should be possible to use it.
There's a bug report for it in Nautilus' launchpad, so maybe Ubuntu will make a patch one day, although I doubt it.