Editor scrolling:

* Make sure the background color for tools is only applied to the main editor.
* Fix toolbar width for smaller screens.
* Ensure toolbar doesn't end up in a weird place when scrolling down too fast.
* Avoid JS errors for post types that don't support the editor.

props avryl. see #28328.

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git-svn-id: http://core.svn.wordpress.org/trunk@28861 1a063a9b-81f0-0310-95a4-ce76da25c4cd