This is something that was answered in the past already for previous versions, there are bug reports but nothing really solves it and it's still present in 16.04
If you have Ubuntu 16.04 Unity as host and Windows guest in Virtualbox VM, when you are on the Windows guest using Super or Alt keys makes that both the guest and unity capture the key. Super key will open the Dashboard in Unity host and Start menu in Windows Guests and Alt key will open the HUD in Unity and any of the uses of Alt in Windows.
There are several bug reports about this. I.e.: https://bugs.launchpad.net/ubuntu/+source/compiz/+bug/741869 https://bugs.launchpad.net/unity/+bug/1060225 ...
and the only workaround is this: How do I prevent Ubuntu from capturing the Alt key to show the HUD?
Which is simply to 'disable' the HUD key in CCSM... not a good solution.
Does someone knows a good workaround or if this is on going to be fixed? This kind of extremely annoying bug, very located, is something that makes me unable to use Ubuntu for work (Yes... I can't be working with VM and having menus of Unity popping up all time)
VirtualBox is widely used, I understand that this is a major issue but in internet I can't find a proper fix. Am I missing anything?
Note 1: I also use Linux Mint 17.3 Cinnamon and this doesn't happen there. Note 2: This happens (that I have tested) with every version since Ubuntu 14.04 Note 3: Checking similar questions about this and the different bugreports, VMWare and any other application (games, etc) that require to use the or keys, have same issue. And there is no intention to fix it in any bugreport.... it's a shame!!