Editor: Merge some minor bug fixes.

There were some tweaks made to the post editor in WordPress 5.0 that hadn't been merged to `trunk`, this commit rectifies that.

Merges [43815,43816] from the 5.0 branch to trunk.

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Fixes #45037.


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git-svn-id: https://core.svn.wordpress.org/trunk@43995 1a063a9b-81f0-0310-95a4-ce76da25c4cd