Notes.app won't stop floating!

Unlike some other posts here, asking how to float a Note in Notes.app I have the opposite problem. When I launch Notes.app, I see the sidebar with the categories, and for the selected category, I see a list of the notes. If I click on a note, nothing happens. If I double click, the note body opens in a separate window.

This is not what I am used to. I want the note body to appear in the same window, in a new column on the right. I don't know how I got here and I have researched a bit and cannot find any mention of this problem!