my-sites-search.php

<?php
/*
 * Plugin Name: My Sites Search
 * Plugin URI: trepmal.com
 * Description: https://twitter.com/trepmal/status/443189183478132736
 * Version: 2016.08.12
 * Author: Kailey Lampert
 * Author URI: kaileylampert.com
 * License: GPLv2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * TextDomain: mss
 * DomainPath:
 * Network: true
 */

add_action( 'admin_bar_menu',        'mss_admin_bar_menu' );
add_action( 'wp_enqueue_scripts',    'mss_enqueue_styles' );
add_action( 'admin_enqueue_scripts', 'mss_enqueue_styles' );
add_action( 'wp_enqueue_scripts',    'mss_enqueue_scripts' );
add_action( 'admin_enqueue_scripts', 'mss_enqueue_scripts' );

/**
 * Add search field menu item
 *
 * @param WP_Admin_Bar $wp_admin_bar
 * @return void
 */
function mss_admin_bar_menu( $wp_admin_bar ) {

	if ( ! is_user_logged_in() ) {
		return;
	}

	$total_users_sites = count( $wp_admin_bar->user->blogs );
	$show_if_gt        = apply_filters( 'mms_show_search_minimum_sites', 10 );

	if ( $total_users_sites < $show_if_gt ) {
		return;
	}

	$wp_admin_bar->add_menu( array(
		'parent' => 'my-sites-list',
		'id'     => 'my-sites-search',
		'title'  => sprintf(
			'<label for="my-sites-search-text">%s</label><input type="text" id="my-sites-search-text" placeholder="%s" />',
			esc_html__( 'Filter My Sites', 'mss' ),
			esc_attr__( 'Search Sites', 'mss' )
		),
		'meta'   => array(
			'class' => 'hide-if-no-js'
		)
	) );
}

/**
 * Enqueue styles
 * Inline styles with admin-bar dependency
 *
 * @return void
 */
function mss_enqueue_styles() {
	ob_start();
	?>
#wp-admin-bar-my-sites-search.hide-if-no-js {
	display: none;
}
#wp-admin-bar-my-sites-search label[for="my-sites-search-text"] {
	clip: rect(1px, 1px, 1px, 1px);
	position: absolute !important;
	height: 1px;
	width: 1px;
	overflow: hidden;
}
#wp-admin-bar-my-sites-search {
	height: 38px;
}
#wp-admin-bar-my-sites-search .ab-item {
	height: 34px;
}
#wp-admin-bar-my-sites-search input {
	padding: 0 2px;
	width: 95%;
	width: calc( 100% - 4px );
}
	<?php
	$style = ob_get_clean();
	wp_enqueue_style( 'admin-bar' );
	wp_add_inline_style( 'admin-bar', $style );
}

/**
 * Enqueue JavaScript
 * Inline script with jQuery dependency
 *
 * @return void
 */
function mss_enqueue_scripts() {
	$script = <<<SCRIPT
jQuery(document).ready( function($) {
	$('#wp-admin-bar-my-sites-search.hide-if-no-js').show();
	$('#wp-admin-bar-my-sites-search input').keyup( function( ) {
		var searchValRegex = new RegExp( $(this).val(), 'i');
		$('#wp-admin-bar-my-sites-list > li.menupop').hide().filter(function() {
			return searchValRegex.test( $(this).find('> a').text() );
		}).show();
	});
});
SCRIPT;

	wp_enqueue_script( 'admin-bar' );
	wp_add_inline_script( 'admin-bar', $script );
}