I'm using Thunderbird 68.0 (build id 20190816191614), with its own profile, under Windows 10. It initially had no message pane, displaying the welcome pane instead. I enabled the menu toolbar.
I enabled the message pane in Options -> Layout -> Message Pane using the hamburger button. That appears to create a message pane with no lines (zero height). It should have created a message pane with the same height as the welcome pane.
I looked at View -> Layout and it believes there is no message pane. If I drag and drop the bottom edge of the folder listing window upwards that creates a working message pane and updates View -> Layout in the toolbar to indicate there is one.
If I then drag and drop the top of the message pane to the bottom of the folder listing (zero line height) both View -> Layout and Options -> Layout in the hamburger button believes there is no message pane. If I drag and drop the bottom edge of the folder listing window upward to create a working message pane View -> Layout believes it exists but Options -> Layout in the hamburger button doesn't.
View -> Layout in the toolbar and Options -> Layout in the hamburger button are not working together correctly.
I did not have this problem using version 60.8 (with its own profile)