Close First Sidebar (widget area) by default.

wp-admin-widgets_close_first_by default.php

<?php
// don't add again


/** 
 * Close First Widget Area by default
 * @modified from: https://wordpress.stackexchange.com/a/114724/64742
 */
add_action( 'admin_footer-widgets.php' , function(){
    ?>
    <script type="text/javascript">
    jQuery( document ).ready( function( $ ) {
        $( '#widgets-right .widgets-holder-wrap:first' ).addClass( 'closed' );
        $( '#widgets-right' ).css( 'opacity', '1' );
    } );
    </script>
    <?php
} );

add_action( 'admin_head-widgets.php', function(){
    echo '<style type="text/css">
    #widgets-right {opacity:0;transition: opacity .5s ease-out}
	.no-js #widgets-right {opacity: 1}
    </style>';
} );