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

git-svn-id: https://develop.svn.wordpress.org/trunk@29978 602fd350-edb4-49c9-b593-d223f7449a82