Comment by Dwedit

When you have a dialog window too big for the screen, you can Alt+Space to open the system menu, then activate the Move menu item, then you can use arrow keys to move the window around, even with the title bar out of bounds.

Not just for obsolete systems, sometimes a full screen application might pick a tiny desktop resolution as well, and not properly restore the resolution, so you could need to deal with a too-big dialog box in that situation as well.