Shortcut identifiers (underlined letters) in window menus are not shown when no item is highlighted

[hife]

Release: Ubuntu 16.04.2 LTS
Package version (unity): 7.4.0+16.04.20160906-0ubuntu1

Expected behaviour: Shortcut identifiers in window (sub-) menus are shown exactly when they are applicable.

Current behaviour: Shortcut identifiers in a (sub-) menu are shown exactly when an item of this particular (sub-) menu is highlighted.

Problematic case: When accessing a menu via Alt+[letter] (e.g. Alt+F for File) or when switching between top-level menus using arrow keys, no menu item will be highlighted, so the actually active shortcuts of that menu are not underlined. You have to first use the mouse or the arrow keys to highlight a menu item.

This defeats the purpose of these shortcuts to select an item without having to use the mouse or arrow keys to go through the menu.