This prevents the editor from jumping off screen when the rendered visual content is significantly taller than plain text, e.g. when there are views present.
This is also a close restoration of 3.9 behavior, some of which was browser-specific.
props avryl.
fixes#29338.
Built from https://develop.svn.wordpress.org/trunk@29693
git-svn-id: http://core.svn.wordpress.org/trunk@29468 1a063a9b-81f0-0310-95a4-ce76da25c4cd