Ticket #29979: twenty-fifteen.php

<?php
/*
Plugin Name: Twenty Fifteen Scroll
*/

function tf_script() {
        ?>
        <script type="text/javascript">
        jQuery( document ).ready( function ( $ ) {
                $( '#sidebar' ).css( 'overflow', 'hidden' );
                
                var sidebar_height = $( '#sidebar' )[0].scrollHeight; //608
                var content_height = $( '#content' )[0].scrollHeight; //582
                var ratio = sidebar_height / content_height;
                
                $( window ).scroll( function() {
                        var content_position = $( window ).scrollTop();
                        var sidebar_position = content_position * ratio;
                        $( '#sidebar' ).scrollTop( sidebar_position );
                } );
        } );
        </script>
        <?php
}
add_action( 'wp_head', 'tf_script' );


function tf_add_jquery() {
        wp_enqueue_script( 'jquery' );
}
add_action( 'wp_enqueue_scripts', 'tf_add_jquery' );