User Preferences window replacing 3D window

Today, when I hit “command-,” or choose “file, user preferences”, instead of creating a new window, it replaces my current working window with a small user preferences window, closing the 3D window. I have tried changing that window to a 3D window, then hitting “command-,” again, but it switches the current window back to a user preferences window. Also, if I choose “User Preferences” while in full-screen mode (Mac, Yosemite) it creates a user prefs window, leaves the rest of the screen black, and cannot be exited except by quitting Blender.
I just updated to the new version of Blender, but that didn’t change the behavior at all!

could you share a screenshot?

Okay, so here are two screenshots. One shows the regular window, and the other shows what happens when I choose User Preferences. It switches the window to User Preferences, and collapses it. In order to go back to work, I have to rebuild my workspace. If I do it while in full-screen mode, it leaves the desktop black, and the window cannot be resized to full size without quitting Blender.

Possibly it’s related to graphics drivers, but also, where is your top menu?

It keeps disappearing. Then I have to manually put it back.