Admin menu:
- Fix scrolling the pinned menu with a mouse wheel.
- Fix pinning when the menu is only slightly taller than the viewport.
- Disable pinning on IE8, updating CSS top makes it jump when scrolling with a mouse wheel.
See #29806
Built from https://develop.svn.wordpress.org/trunk@29978


git-svn-id: http://core.svn.wordpress.org/trunk@29724 1a063a9b-81f0-0310-95a4-ce76da25c4cd