Thanks Ann, that explains why I can’t find it! 

What I’m wanting to do is disable Shift+2 from scrolling through the clips in the source monitor. I still want Shift+2 to switch focus to the Source Monitor, but I don’t want a second/subsequent press of Shift+2 to cycle through the loaded clips. I never use this ability, but it happens often-enough that I accidently press 2 twice, or focus is already on the Source Monitor when I press Shift+2, and the source monitor starts cycling through unwanted clips. I then have to switch to the mouse to get it back to where I want it, disrupting my flow.

Previously, I had managed to disable this behavior. Shift+2 would switch focus to the Source Monitor, and repeated presses of Shift+2 wouldn’t cycle through past clips. But I had to clear my preferences a few months ago to get rid of some bugs, and now it’s back to the default behaviour and I cannot work out how to get back to the way I’d configured it.