There is no way to configure mouse settings in KDE Neon dev-unstable, running under VirtualBox. For exmaple, no way to setup double-click to open folders, had to manually edit ~/.config/kdeglobals and add SingleClick=false to [KDE] section there. https://i.imgur.com/NUTiUOZ.png - no "Mouse" section, and some options like using double click to open files in unconfigurable.
Is this still an issue? I do not have a problem under KVM.
I re-tsted it with VirtualBox: - latest Neon dev-unstable 16.04 has Mouse settings back - latest Neon dev-unstable 18.04 also has Mouse settings in place In both cases single/double click setting to open folders was moved into "Desktop behavior / Workspace" section. Originally this issue was related to some mouse kcm rework (I think, I don't remember exactly)