Unable to trigger hidden panels/bars on any screen edge
I've found it pretty much impossible to use Boxes to test auto-hiding panels or top bars in any Linux distro or desktop environment that I've tried.
I like to have panels auto-hide and get out of my way. Ubuntu Dock, Plank Dock, the top bar of GNOME, the main panel in KDE or MATE or Cinnamon or Xfce; they pretty much all have the option to automatically or intelligently hide themselves along a screen edge. In bare metal installs these features normally work fine. But inside Boxes, whether in windowed or fullscreen mode, it seems to be impossible to trigger any kind of hidden panel, dock or bar anywhere on the screen. At the top of the VM window the Boxes header bar interferes in most cases, but hidden docks and panels also won't pop up on the left, right or bottom edges of the screen.
This makes it very difficult to test any desktop environment that has any feature like this.
Boxes version 40.0 is what I've been working with, on multiple distros like KDE Neon and Ubuntu 20.10.