Disappeared preview pane in Mail v.13.4

Using Catalina v.10.15.7. No problems until now.

In Mail, the preview pane has disappeared and I can't find it either to the side or the bottom. I click on an incoming email, and there is no preview. "Show side preview" is checked under the VIEW menu. Mail Help says there should be an option for bottom preview, but I don't see a bottom preview and I don't see an option for it.

Other users seem to have had similar problems over the years, but I can't find a single helpful answer. I tried rebooting in Safe mode, but that did not solve it. Hoping for an answer! In the big scheme of things, this is not a huge problem, but it is annoying.