Q: mail viewing window stuck

I'm using Mail with El Capitan (OX10.11.8) and I have always preferred to display message titles only, not previews, in the viewing window. However, recently, I pulled the viewing window all the way down to search for a certain message, and it will not go back up.

When I click on the dot at the bottom of the frame, the cursor turns into a straight horizontal line with an arrow pointing upward and does... nothing.

Why should I have been able to pull the window down and not back up? I have tried restarting Mail and restarting the computer, so if that's your suggestion, thanks, but no thanks.

I want my message view below the mail list back! None of the menu items seem pertinent.