It would be a great feature if we can advertise full out-of-the-box Global Menu support in Cosmic. I've been collecting information about how we can add support by default and here's what I've come up with so far:
1. Install these packages by default:
```
# For GTK app support
appmenu-gtk
appmenu-gtk-module-common
appmenu-gtk2-module
appmenu-gtk3
appmenu-gtk3-module
libappmenu-gtk2-parser0
libappmenu-gtk3-parser0
# For non-KDE Qt app support
appmenu-qt
# For Electron app support
libdbusmenu-glib4
# For LibreOffice support
libreoffice-gtk3
# Should already be installed by default as it's a part of Plasma in 5.13 and later
gmenu-dbusmenu-proxy
```
2. If we don't ship with LibreOffice 6.2 or greater, also set some more environment variables:
```
export SAL_USE_VCLPLUGIN=gtk
export GTK_MODULES=appmenu-gtk-module
```
3. Possibly take this patch: https://cgit.kde.org/plasma-workspace.git/commit/?id=c9ba8e0cf7e518de95952762dc56d930df98339f (in 5.14 only; supposedly fixes an issue with LibreOffice, need to confirm)
4. Possibly also install all `*unity-gtk-*` packages as well (need to confirm)
Note: I haven't tested any of this yet.