If you like to use the function :
Always remember that on new jQuery 3.5.1 this is deprecated

$(window).load(function(){

and you had to use now:

$(window).on('load', function(){

if you are talking about the logo that could be seen on transparency header – this is the one in .subtext
may i see the page it concerns too. I’m participant as you are – so private Content area can not be seen by me.

If you see the demo page : https://kriesi.at/themes/enfold-2017/
and on mobile view there will be no transparency – that seems to be the default behavior – but on my home page ( test page ) i got this too.
on mobile view there will be transparency first – after scroll there will be a fixed white header then.
https://webers-testseite.de/
( the only difference here is that these imgs are replaced by svgs )

So is i want to change only the transparency logo on home page – i had to replace the logo in the subtext ( alternate class )

yes – but think of jQuery 3.5.1 : jQuery(window).load(function(){ is deprecated now.
You had to use instead: jQuery(window).on('load', function(){

or here in the code: Link

$(window).on('load', function(){
…

and if it is the syntax is:

(function($){
…
})(jQuery);

you can write every jQuery inside as $: ( f.e. $(‘body’) }

function av_open_inline_popup(){
?>
<script type="text/javascript">
(function($) {
$(window).on('load', function(){
	$('.open-popup-link').addClass('no-scroll');
	$('.open-popup-link').magnificPopup({
		type:'inline',
		midClick: true,
		callbacks: {
			beforeOpen: function () {
				$('body').css("overflow-y", "hidden");
			},
			close: function() {
				$('body').css("overflow-y", "auto");
			},
		},
	});
});
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'av_open_inline_popup');

i think no : Link

PS : on Iconlist the same – lines 892ff
enfold/config-templatebuilder/avia-shortcodes/iconlist/iconlist.php

Similar way – but with shinking header in action:
https://webers-testseite.de/xeovision3/

___________________
adjust it to your page-id !

#top.page-id-40389 {
  background-size: 0 !important;
  overflow: hidden;
  position: relative;
}

#top.page-id-40389:before {
  background-image: url(/wp-content/uploads/nature-3082832_1920.jpg);
  background-repeat: no-repeat;
  background-size: cover;
  content: "";
  position: fixed;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  z-index: -1 !important;
  will-change: transform;
  pointer-events: none;
}

#top.page-id-40389 #wrap_all {
  background-color: transparent !important
}

#top.page-id-40389 #main {
  background-color: transparent !important
}

#top.page-id-40389 .avia-section.transparent-section {
    background-color:  transparent !important;
}

The trick via before pseudocontainer is that it is a container set to fixed position – and not that body has a background-attachment : fixed. some Browser do react on that with jitter (Safari)

hm – isn’t it solved ?
why another topic for it?

https://kriesi.at/support/topic/color-section-parallax-image-scaling/#post-1307972
https://webers-testseite.de/xeovision/

with the changes above it will be:
( click to enlarge )

for example you can change those lines to :

if( ! empty( $link ) )
{
	$linktitle = $title;

	switch( $linkelement )
	{
		case 'both':
			if( $title ) 
			{
				$title = "<a href='{$link}' title='Link to " . esc_attr( $linktitle ) . "' $blank>$linktitle</a>";
			}
			
			$display_char_wrapper['start'] = "a href='{$link}' title='Iconlink to "  . esc_attr($linktitle) . "' {$blank}";
			$display_char_wrapper['end'] = 'a';
			break;
		case 'only_icon':
			$display_char_wrapper['start'] = "a href='{$link}' title='Iconlink to " . esc_attr($linktitle) . "' {$blank}";
			$display_char_wrapper['end'] = 'a';
			break;
		default:
			if( $title ) 
			{
				$title = "<a href='{$link}' title='Link to " . esc_attr( $linktitle ) . "' {$blank} >$linktitle</a>";
			}
			
			$display_char_wrapper['start'] = 'div';
			$display_char_wrapper['end'] = 'div';
			break;
	}
}

see results here on those two iconboxes on the right: https://webers-testseite.de/tagood/

this is the layout i used for it:
( click to enlarge )

here is the css code for it in quick css:
( in my case it is that page-id – adjust to your page-id )

#top.page-id-40318 #main {
  overflow-x: hidden;
  height: 100vh;
}

#top.page-id-40318 .avia-section.xeovision {
    position: -webkit-sticky;
    position: sticky;
    top: 0;
    height: 100vh;
    display: flex !important;
    justify-content: center;
    align-items: center;
    letter-spacing: 0.2em;
    color: white;
    background-size: cover !important;
    background-position: center !important;
}

/*** and everything that follows the xeovision section  f.e.: a grid-row (.av-layout-grid-container) ***/
#top.page-id-40318 .avia-section:not(.xeovision) {
    position: relative; 
}

@xeovision: fixed: the element is removed from the flow of the document like absolutely positioned elements. In fact they behave almost the same, only fixed positioned elements are always relative to the document, not any particular parent, and are unaffected by scrolling.
I think that you will not succeed with a fixed section. However, it would be better to see your page live where you tried it. I can then rather give advice on how it might be done.
However, it may be enough as it is here: https://webers-testseite.de/xeovision/
but for a transparent color-section following – the top section had to be 100% screen-height. (100vh)

Or more tricky : background image to html ;)
and transparencies where ever you like : https://webers-testseite.de/xeovision2/

the calculation of the height is on shortcodes.js on line 285ff:

_fetch_properties: function()
{
	this.property.offset 	= this.$parent.offset().top;
	this.property.wh 		= this.$win.height();
	this.property.height 	= this.$parent.outerHeight();
	
	//set the height of the element based on the windows height, offset ratio and parent height
	this.$el.height(Math.ceil((this.property.wh * this.ratio) + this.property.height));
	
	//re-position the element
	this._parallax_scroll();
},

you see that the scroll ratio comes into account.
If your scroll ratio will be less than the default 0.5 the height will result in a less height.
you can influence the scroll speed by this snippet in child-theme functions.php:

add_filter('avf_parallax_speed','avia_change_parallax_ratio', 10, 2);
function avia_change_parallax_ratio($ratio, $id){
$ratio = "0.2";
return $ratio;
}

but then the parallax effect is less impressiv – but the background-images won’t be cropped as much as a 0.5 ratio will do.
There is one more thing to clarify where is the influence of the scroll offset and wouldn’t it make sense to include the height of the header in the calculation if it is fixed.
I use a fixed header quite gladly, and also already times with 150px height and more. So such an image would not have to have height.
this.$el.height(Math.ceil((this.property.wh * this.ratio) + this.property.height));

Leider kein Private Content für mich. Bin selbst hier nur Teilnehmer.
Kannst mir aber gerne ne E-Mail schicken. Infos unter meinem Nick oder Avatar.

PS: bislang sehe ich keine Änderung. Der erste Aufruf der Seite – ohne auf das popup unten zu reagieren! – Seite mit Developer Tools gecheckt!
Analytics immer noch aktiv. Ein OptOut ist nicht zulässig!
Sonst habe ich ja Borlabs Cookies am Start, aber es geht auch mit Enfold: https://webers-testseite.de/weber/
Du musst da auswählen:

If you want to keep the aspect ratio, the height determines the enlargement factor if you are using landscape images.
Therefore, I recommend using a different image in portrait format than in landscape format.This avoids that important content is cut off.
________
Wenn du die Seitenverhältnisse beibehalten willst, bestimmt die meist die Höhe eines Querformatbildes den Vergrößerungsfaktor.
Daher empfehle ich dann im Hochformat ein anderes Bild einzusetzen als im Querformat. Hierdurch wird vermieden, dass Bildwichtige Inhalte beschnitten werden.

there are two if clauses to add. I told you above that the svg attribute of preserveAspectRatio decides how the svg react on shrinking.
so if your logo is on the right side ( a class exists on html : html_logo_right – and if the logo is centered it has on html the class: html_logo_center
so code of changing all img svgs goes to inline-svgs has two check if img is a logo img and if html got this class – and change that attribute:

// replace every svg with its inline code
function replace_img_if_svg_with_inline_svg() {
?>
<script type="text/javascript">
(function($) {
  $('img').filter(function() { return this.src.match(/.*\.svg$/); }).each(function(){
    var imgSVG    = $(this);
    var imgURL    = $(this).attr('src');
    var imgAlt    = $(this).attr('alt');
    var imgTitle  = $(this).attr('title');
    var imgClass  = $(this).attr('class');
    $.get(imgURL, function(data) {
		var svg = $(data).find('svg');
		// Remove invalid XML tags
		svg = svg.removeAttr('xmlns xmlns:xlink');
		// add replaced image's alt tag to the inline SVG
		typeof imgAlt === 'undefined' || imgAlt === '' ?  (svg = svg.attr('alt', 'Replaced Image')) : (svg = svg.attr('alt', imgAlt)) ;
		// add replaced image's Title tag to the inline SVG
		typeof imgTitle === 'undefined' || imgTitle === '' ? (svg = svg.attr('title', 'A SVG Image replaced by its inline code')) : (svg = svg.attr('title', imgTitle));
		// Add replaced image's classes to the new SVG and add replaced-svg as new class
		typeof imgClass === 'undefined' || imgClass === '' ? (svg = svg.attr('class', 'replaced-svg')) : (svg = svg.attr('class', imgClass+' replaced-svg'));
		// check where the logo is placed : left or right or centered in the header
		if ($(imgSVG).parents('.logo').length && $('html').hasClass('html_logo_left')) {
			svg.attr('preserveAspectRatio', 'xMinYMin meet');
		};
		if ($(imgSVG).parents('.logo').length && $('html').hasClass('html_logo_right')) {
			svg.attr('preserveAspectRatio', 'xMaxYMin meet');
		};
		if ($(imgSVG).parents('.logo').length && $('html').hasClass('html_logo_center')) {
			svg.attr('preserveAspectRatio', 'xMidYMin meet');
		};
		// Replace image with inline SVG Code
      	imgSVG.replaceWith(svg);
    }, 'xml');
});
})(jQuery);
</script>
<?php
}	
add_action('wp_footer', 'replace_img_if_svg_with_inline_svg');

you can see here on my test-page that if the logo is on the right – it shrinks to the right:
https://webers-testseite.de/impressum/datenschutzerklaerung/
here is the css code for “nearly” all instances ( header left, center, right with transparency or not ):

.logo, .logo a {
  overflow: visible;
}

.logo a {
  display: flex;
}

#top .logo svg {
  position: relative;
  left: 0;
  top: 0;
  width: 450px;
}

#top .logo .subtext svg {
    position: absolute;
}

.html_logo_right #top .logo svg {
  left: auto;
  right: 0;
}

.html_logo_center #top .logo > svg {
  -webkit-transform: translateX(-50%);  
  transform: translateX(-50%);
}

#top .av_header_transparency.av_alternate_logo_active .logo a > svg {
    opacity: 0;
    filter: alpha(opacity=0);
}

#top :not(.av_header_transparency).av_alternate_logo_active .logo .subtext svg {
    opacity: 0;
    filter: alpha(opacity=0);
}

.av_header_transparency .logo .subtext svg {
    opacity: 1; 
}

Edit – the menu is very nice – i now see that the first sublevel Links goes under the main navigation ( mybe that is the reason for the middle positioning)
wow. that will be hard to reach with enfold only options and css.

on website opening in a lightbox the ?iframe=true is ok. See: https://webers-testseite.de/my-website/
For images as Yigit said it is not necessary.
( you can see that on enfold/js/avia-snippet-lightbox.js on line 207ff)
the groups and autolink elements do open in lightbox without any addons.