This is not a new idea - it's a REQUEST to go back to the old versions where you could reduce the 'Message List Pane' right down to 2 or 3 lines. Now, on my screen, it's a MINIMUM 10 lines, so I can barely read my emails. It's just utterly pointless, and there are now DOZENS of comments/complaints asking for this to be reverted. PLEASE do so . . .
There is a bug report on this topic and I've no idea why the developers have not fixed this issue because it would take a matter of minutes but if you want a fix then you need to follow the instructions at this link:
Yes, it means using 'userChrome.css', but it's not like anyone has to know how to code.
The truth is this, if you have the following skillset then it's very simple.