As of VirtualBox 6.0, they should be able to coexist (because VirtualBox piggies onto Hyper-V when it’s enabled effectively).

In practice it's not really working though. At least not yet. There's a very long thread on virtualbox forums with people trying to get it to work but failing.

I personally have to reboot when I need to use docker or virtualbox. Very annoying.

Yep, this is currently confirmed broken on Windows 10 1903 (Windows Hypervisor Platform extended user-mode APIs). Affects VirtualBox, qemu, etc. We may see a patch, but it's up in the air right now. (<closes eyes and sighs>, I know.)

Qemu? Qemu does processor emulation AFAIK.

But then it is really Hyper-V with a VirtualBox UI. Seems like a recipe for incompatibilities etc.

The team has said its working with VMware and VirtualBox on solutions but I have no idea what those will be.

Hyper-V supports nested virtualization (only Hyper-V nested inside th Hyper-V, though), and seeing as VirtualBox supports a Hyper-V virtualization backend, the solution may be to just find and fix all the bugs in that backend code when under Hyper-V

Unless virtual box utilizes hyper v, you'll have to reboot to switch between