Automatically open selection in other editor - not working as expected

I have chosen the “Open Selection in other editor” (the icon in the footer) and it seems not to be working as expected.

I suspect I have changed a setting to override it but can’t find it. At the moment, when I have the icon selected, it opens documents in the current editor.

What I want to do and have successfully done before (in S3 even) is to highlight the Research section in the current editor and have all its documents appear in the other editor.

Your help is much appreciated.

OK, I just opened another project in S3 and confirmed that this works as I want (I highlight the Research or Corkboard folder, and each of that folder’s documents open in the other editor.

But I still can’t figure out what I have done in the other project to override this capability.

In the first project, have you locked the other editor, by any chance?