A stupid question about synchronization

I'm sure this question is stupid. I have used Opus for many years on my computers.
I should mention that I use a view that shows an upper panel and a lower panel.
Now, suddenly, I see that my latest installation of Opus on a new computer seems to want to make the upper panel the source and the lower panel the destination. But, on all my other machines, if I select one of the panels, then that panel because the source and the other is the destination.
What am I overlooking.
Many thanks
Jonathan Lewin

There's a Sync Left-to-Right checkbox in the sync panel which locks the source/dest sides. (Left = Top, Right = Bottom, if using a horizontal layout.)