Footer that appears on scrolling

this is called curtain effect and you can try this:
https://kriesi.at/support/topic/fixed-curtain-footer/#post-691135

see result here ( only starting page ) : https://webers-testseite.de/pureinstall/

on my testpage i use this in child-theme functions.php:

// curtain effect of footer - 
function add_curtain_footer_effect(){
if(is_home()){  
?>
<script type="text/javascript">
(function($) {
    $(window).bind("load resize", function() {
	    setTimeout( function() {
			var socketh = $('#socket').outerHeight();
			var footerh = $('#footer').outerHeight();
			var spacerh = socketh + footerh - 5 ;

			$('#distance').css('height', spacerh );
			$('#footer').css('bottom' , socketh);
	    }, 150) 
    });
})(jQuery);
</script>
<?php
}
}
add_action('wp_footer', 'add_curtain_footer_effect');

and on quick css ( pay attention as the script above this is all only for home – so adjust to your needs):

.home #main > div  {
    z-index: 3 !important;
    position: relative !important;
}

.home #footer { 
    left: 0;
    width: 100%;
    z-index: 2;
   position: fixed
}

.home #socket {
    bottom: 0;
    left: 0;
    width: 100%;
    z-index: 2;
    position: fixed
}

.home #main #distance  {
    clear: both;
    z-index: 0 !important;
}