I tried that yesterday. No luck. The red, yellow, and green "dots" did not return. Having read what you posted above, I did Shift-F again. Still no luck. Then I started doing it over and over again. The Lightroom window suddenly shrank slightly, so it was no longer full screen, and the three "buttons" re-appeared.
So, first comment - Problem Solved. Thank you!
New question - why didn't this work the first few times I did this? When I did the Shift-F over and over again, the reduced size Lightroom window switched back and forth to showing Lightroom, and then showing the single image that was selected "full size" within that window.
I guess I need to look up the effects of "F", "Shift-F", and I think there were other key combinations. I think I will avoid making the window full-screen from now on, until I understand this better.
Again, thank you!
Thanks for asking this question - happened on my Mac desktop as well, thought I was going crazy. Starting to keep a running list of little helpful tips like this.