Ticket #29979: twenty-fifteen.2.php

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

function tf_script() {
        ?>
        <script type="text/javascript">
        jQuery( document ).ready( function ( $ ) {
                $( '#sidebar' ).css( 'overflow', 'hidden' );
                
                var sidebar_scrollheight = $( '#sidebar' )[0].scrollHeight;
                var content_scrollheight = $( '#content' )[0].scrollHeight;
                var ratio = sidebar_scrollheight / content_scrollheight;
                var window_height = $( window ).height();
                var offset = content_scrollheight - window_height;
                var multiplier = content_scrollheight / offset;
                
                $( window ).scroll( function() {
                        var content_position = $( window ).scrollTop();
                        var sidebar_position = Math.round( content_position * ratio * multiplier );
                        $( '#sidebar' ).scrollTop( Math.abs( 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' );