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

wp-admin/js/customize-controls.js

@@ -1867,6 +1867,9 @@
 				// Search terms.
 				debounced = _.debounce( section.checkTerm, 500 ); // Wait until there is no input for 500 milliseconds to initiate a search.
 				section.contentContainer.on( 'input', '.wp-filter-search', function() {
+					if ( ! api.panel( 'themes' ).expanded() ) {
+						return;
+					}
 					debounced( section );
 					if ( ! section.expanded() ) {
 						section.expand();

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-42185';
+$wp_version = '5.0-alpha-42187';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.