Post UI: Increase the trigger width for switching from two columns to one.

props melchoyce, vinod dalvi.
fixes #23904.
Built from https://develop.svn.wordpress.org/trunk@25688


git-svn-id: http://core.svn.wordpress.org/trunk@25604 1a063a9b-81f0-0310-95a4-ce76da25c4cd