MailMate: inverted three-pane view
Just discovered that MailMate (for macOS) lets you write custom views. Here is a trivial one which flips the normal three-pane view so that the message viewer is above the message list, rather than being stuck in the bottom third of the screen. It's based on a default as described in this blog post.
To use it, download messageabove.plist, rename it so it ends in .plist, copy it to ~/Library/Application Support/MailMate/Resources/Layouts/Mailboxes (which you will probably need to create), restart MailMate, and select the new layout from View ➤ Layout.