When the submenu is too long to fit on the screen, place the top if it under...

When the submenu is too long to fit on the screen, place the top if it under the admin bar (the rest is scrollable with a wheel mouse), fixes #19323

git-svn-id: https://core.svn.wordpress.org/trunk@19487 1a063a9b-81f0-0310-95a4-ce76da25c4cd