I was running Fedora on VirtualBox and I encountered a problem where the dialog box was too big to fit in my virtual screen and there was simply no way to drag the window up so I could see the buttons at the bottom of the dialog box! Once the title bar of the dialog box hit the top of the screen, the window could move no further.
As usual, performing a search on the Internet provided a quick solution. Just hold on to Alt while clicking and dragging anywhere within the dialog box and it will then magically be able to move beyond the screen.