I upgraded from Ubuntu 16.04 to 16.10, and have noticed that I now have an unresponsive Ubuntu menu bar when I have VirtualBox in window-mode. The top-level menu items display (ie,File, Machine, View, etc), and each of them highlight when clicked, but they do not show their contents / submenu items. Additionally, my indicators are all unresponsive.
The menu bar works fine when VirtualBox is not focussed, and the mini-toolbar works fine when VB is in Seamless or Fullscreen mode. When in Seamless mode, the Ubuntu menu bar is completely missing, although it reappears when I focus the mini-toolbar.
This is with Ubuntu 16.10, and VB 5.1.14 r112924, although the problem also occurred with 5.1.12.
It might be time for a fresh host OS install. Is this just me?