My host OS is Ubuntu 14.04 and guest OS in VirtualBox is Windows 8.1. I can switch from host to guest by Alt+Tab, but can't from guest to host by the same method which only switches between windows in guest OS. How can I switch from guest to host? Thanks!
Btw, I still would like a way to switch between windows in guest OS.
This is caused by the fact that Virtualbox "captures" keystrokes for the guest OS. You can disable this momentarily by pressing the "Host" key, which normally is the right Ctrl button.
Try pressing the right Ctrl button and then Alt+Tab-ing, you should go back to the host OS.
How to switch to host:
For me (Windows 10 host, Ubuntu guest) actually works the following: pres&release right Ctrl, then Alt+Tab
The switching function you are looking for is achievable in VMWare Player, via unity mode. At the moment VirtualBox doesn't offer this option.
Anyway, VMWare Player 12 is now available to be used under Ubuntu 15.10. It works.