I just pushed "layout modes" change into dev-layouts branch. This is an implementation of three column view for mail UI.

This is a BC breaking change. So, if you're an author of a skin or a plugin that modifies the messages list, you'll most likely have to do some modifications of your code.

You can also just start beta-testing of this change, so it will be ready for 1.3-beta release. Have fun!

https://kolabian.wordpress.com/2016/09/17/widescreen-layout-aka-three-column...