Suddenly, when I split the editing window, I can't show separate documents

All of a sudden, when I split my editing screen into two, I can no longer view separate documents. Both screens are selected, and when I choose a binder document for the left screen, it shows up in the right screen as well. They automatically “match.” How do I turn the “matching” off?

Hi

Top menu : Navigate / Binder selection affects >
Likely you are now set to “Both”.