Cannot adjust reading window in Mail.app classic view

The slider that should adjust the two is at the bottom of the window. Hovering over it changes it to the upside down T that should be draggable. Unfortunately its stuck and I cant drag it upwards, either with trackpad or mouse.

Drag it farther than you have been doing, much farther. There will be no visual confirmation of that action until you drag it more than halfway to the opposite edge of the window, or full screen if you are not using windowed mode.

I've moved my divider to the bottom of the window. You're right that is is reluctant to move back up again. After several attempts it does move however. I suspect the 'active area' for dragging is too small so the horizontal of the 'T' needs to be in just the right horizontal position. Resizing the window first seemed to help sometimes.

C.

I am having the same problem.

I had been using Classic Layout with message previews. Didn't change anything. After last MacOS Sierra update, started having several problems.

One that occurred a couple of days ago:

Mail view layout changed - now there are just two panes: the narrow mailbox list on the left, and the list of message previews on the right. The third pane, usually on the right, for viewing each message is gone. I can only view messages by clicking on them to open the message in its own window.

The box for Classic Layout had become unselected in Mail Preferences Viewing. When I click on the box to try to go back to Classic, the message list previews disappear. I am wondering if I could/should reinstall the Mail app.

Any ideas?

Thanks in advance.