Enabling 3D graphics for a Windows 10 guest VM on Virtualbox would cause unpredictable visual effects and transparent start menu.
I was running Virtualbox on a Linux Mint 19.3 host and using the NVIDIA driver version 430, which was the default at the time. I tried installing the latest available version, 440, but the issue persisted.
The issue was finally solved by actually going back to an old version, specifically 390. Enabling 3D graphics on my Windows VMs now works perfectly.