My standard workflow for a couple years now has involved using a Debian 8 64-bit VM with Guest Additions on my Windows 8.1 64-bit host and essentially treating it as a terminal emulator, then using my host machine for everything else. I typically have the VM fullscreened and use alt+tab extensively for switching between it and my host.
After I upgraded VirtualBox on Friday to 5.1.18, I am no longer able to alt+tab away from my VM, whether in fullscreened, windowed, or in seamless mode. I can confirm that the host key is working fine and that the problem is not that the keyboard is being unexpectedly captured. Even more bizarre, I also encounter the problem on my 32-bit Debian VM and my Ubuntu 16.04 64-bit VM, but not on my Windows 7 (32-bit) or 10 (64-bit) VMs.
Any ideas?