This sets the `#adminmenuback` `position` CSS property to `fixed`. Props promz, man4toman. Fixes #47010. Built from https://develop.svn.wordpress.org/trunk@46460 git-svn-id: http://core.svn.wordpress.org/trunk@46258 1a063a9b-81f0-0310-95a4-ce76da25c4cd