Over the years, I have seen requests for the a three column inbox. Since the preference page already is configured that way, shouldn't it be a matter of applying that Javascript/CSS to the inbox?
Would it be possible to add that as an option to the Mailbox View: 3 vertical columns and the current split horizontal?
Some variation of the mockup image attached.
A three column layout was added in Roundcube 1.3 https://www.roundcubeforum.net/index.php/topic,24373.0.html