When using libreoffice inside unshare -m session, the menubar is missing.
In the console output I see these warnings:
** (soffice:9193): CRITICAL **: 10:29:29.171:
void g_lo_menu_insert_section(GLOMenu*, gint, const gchar*, GMenuModel*):
assertion 'G_IS_LO_MENU (menu)' failed
(soffice:9193): Gtk-CRITICAL **: 10:29:29.171:
gtk_menu_bar_new_from_model:
assertion 'G_IS_MENU_MODEL (model)' failed
In my normal session, the menubar appears correctly. Also this used to work before I upgraded from 16.04 to 18.04.