Editor scrolling:
- Properly handle change of sidebar height when opening, closing or hiding postboxes.
- Add a flag when to start pinning. Set it to few pixels more than the initial editor height.
See #28328
Built from https://develop.svn.wordpress.org/trunk@29523


git-svn-id: http://core.svn.wordpress.org/trunk@29299 1a063a9b-81f0-0310-95a4-ce76da25c4cd