Twenty Fifteen: correcting odd scrolling and jumping in IE and no-scroll with window resize.

Props iamtakashi, ocean90, fixes #30632, #30645.



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Twenty Fifteen: Removing the empty changelog.txt.

Fixes #30650.


Built from https://develop.svn.wordpress.org/trunk@30809


git-svn-id: http://core.svn.wordpress.org/trunk@30799 1a063a9b-81f0-0310-95a4-ce76da25c4cd