I have Ubuntu both as guest and as host. I use VirtualBox 4, just downloaded from the official repository. After installing the guest extensions, I got this strange problem:
When I activate the resizable screen mode with Host + C, the GNOME desktop won't resize with the screen. Only the wallpaper gets stretched. I use no desktop effects and haven't changed any driver options yet.
Any ideas how to fix this? Here is a screenshot of the guest machine, demonstrating how it looks like after changing the resolution:
I know how to use scalable mode, but I guess I forgot some configuration in the guest machine to make it work.