Notes has a triple-column view and suddenly it's turned off

A Notes window has three elements:

1. A sidebar with Folder view
2. A list of notes filtered by search and/or sidebar selection
3. An inline edit window for the note you selected in column 2

I switched off #3 and cannot get it back! Where is it in the menus?

I don't mind having notes open in new windows but I like the convenience of viewing a note side by side with a browser.