the other day I wanted to give groovy a try and downloaded a daily iso and installed it using virtual box.
The installation went fine, yet, after restarting I was not able to see anything in order to type the login credentials. The restart or starting anew the machine was fine, yet no login screen was appearing; instead black screen was shown.

At the moment I was trying to shut down the session, behind the window of the shutdown options, I was able to discern the login screen (it was grayed).

By changing the resolution of virtual box from the view options the screen was as it should and I was able to login.
The next time I had anew to change the resolution in order to be able to login.

Just a workaround for those that might face the same problem.