Ticket #29979: twenty-fifteen.5.php

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

function tf_script() {
        ?>
        <script type="text/javascript">
        jQuery( document ).ready( function ( $ ) {
                
                if( $( '#content' ).height() < $( window ).height() && $( '#sidebar' )[0].scrollHeight > $( window ).height() ) {
                        $( '#content' ).height( $( '#sidebar' ).height() );
                }

                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;

                if( $( window ).width() > 967 ) {
                        $( '#sidebar' ).css( 'overflow', 'hidden' );
                        $( window ).scroll( function() {
                                var content_position = $( window ).scrollTop();
                                var sidebar_position = ( content_position >= 0 ? Math.round( content_position * ratio * multiplier ) : 0 );
                                $( '#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' );