"F" isn't the issue. I can't get a screen shot because every time I hit Cmd+3 (or 4 partial) + enter Lr starts preparing a slide show. So, here is what I have: Restarted the Mac Pro without Lr running. Kept all my work on the second monitor. Load Lr there and Lr loaded the Loupe on my main Eizo monitor. The Loupe disappears when I leave the main Lr program. It doesn't leave the screen when running one monitor. The screen capture I wanted to attached showed the Lr screen and View menu. The View menu has the Loupe selected and I cannot change that in the Develop Mod. I can switch it to Grid, Survey, etc in the Library Mod, but, those are all part of the same tool set as the Loupe. As soon as I go back to Develop the selection goes back to Loupe.
Any help will be appreciated. BTW, I'm running OS X 11.2 and Lr CC Release 15.3. This problem was first incurred under Lr 15.2, which was upgraded a few hours ago. I'm stumped.
Update:
All is well again. Killed the program and relaunched with the second monitor turned off. Program worked fine. Tried it again with the second monitor on and all is well. However, the 3 little buttons (close/minimize/maximize) are missing. I think Lr is not playing nice with El Capitan, or vise-versa.