Show menu based on scroll position.

some-script-file.js

( function($) {

    $( document ).ready( function () {

        var primaryNav = $( 'body.home' ).find( 'nav.nav-primary' );
        
        // check initial window size
        if( window.innerWidth > 687 ) {

            primaryNav.hide();

        }
    
        // listen for scroll event
        $( window ).on( 'scroll', function(){

            // if screen size is less than 688, return
            if( window.innerWidth < 688 ) {
                return;
            }

            if ( $( this ).scrollTop() > 150 ){
                primaryNav.fadeIn();
            } else {
                primaryNav.fadeOut();
            }

        });

        // listen for browser resize
        $( window ).on( 'resize', function() {
            
            //  if window is less than 688, just show the menu
            if( window.innerWidth < 688 ) {
                primaryNav.show();
            }

        });

    });

} )( jQuery );