Mini toolbar hides itself after you click on a menu header

[Donuts]

Lubuntu 16.04 x86-64 host. This issue seems to apply to all guest OSes.

I have configured guest VMs to show the mini toolbar at the top of the screen when in full-screen mode. If you mouse over the hidden toolbar it appears. On clicking a menu header, the menu appears but the toolbar then hides itself after a second or two. I think the toolbar should remain shown until either the user selects a menu item or clicks away from the menu.