Anyway, quite recently I built a Gentoo system with the new KDE / Plasma 5 environment and I ended up installing the old Xorg in spite of Wayland and all. What I noticed is that Windows in the guest OS disappeared, so did also the system panel bar at the bottom and its menu. Even the mouse cursor disappeared. The setup was pretty much useless. I had it explained to me that it probably is having 3D acceleration that is broken in VBOX, that is the issue. I found it strange because KDE4, GNOME3, Unity et al just work fine with 3D acceleration enabled in VBOX!
Then a developer in the KDE team stepped forward and laid it all out to me in a truly vehement confession:
That moment of clarity was unforgettable... So it seems that newer desktop environments don't work with VBox 3D acceleration. After disabling the acceleration, the environment works fine although it is a bit sluggish compared to when the acceleration is enabled. And it is VBox 4.3.30 I'm using.KDE5 makes *much* more use of *much* more advanced OpenGL in *many* more GL contexts (QtQuick being the main driver here, but we also removed the OpenGL 1.3 backend from KWin) - it's very likely that this knocks out the virtualbox GL driver, yes.
Another issue that is much more prevalent when auto resize guest screen is enabled with 3D acceleration disabled is that the VM window keeps minimising and positioning itself in upper left corner. The only modes that are usable are fully screen and maximised window.