Help! How do I make the 2nd monitor show what I clicked in the library grid?

Before (2 weeks ago), when I was in grid mode in the library view, if I clicked an image, it would show on my second monitor. This was a super fast way for me to quickly go through my images and get rid of ones that were similar, picking the best.

Now, my second monitor view -only- changes by clicking on the filmstrip in the library, irrespective of what I click in the grid.

I really want to have my second monitor show the image that I clicked from the library grid. How do I "put that back"