Customize: Prevent themes panel from auto-expanding in IE11.

In IE11 the `input` event erroneously gets triggered initially without any user input.

Amends [41648].
See #37661.
Fixes #42556 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42187


git-svn-id: https://core.svn.wordpress.org/trunk@42017 1a063a9b-81f0-0310-95a4-ce76da25c4cd