beginners questions serie: background page and lightbox should be fixed position

Dear Cedric,
Thank you for your engagement and your very quick replies!

Hi
regarding your other post on the problem for mobile, please try to modify the code like that

I did follow your advice. Now it looks good on desktop. But it is very different looking and not acceptable on a real smartphone! I suppose, that there is not only a query for screen width but also for the user agent.

As I wrote in the other thread, I have a sample, which is working well. But it runs on an old Joomla 3 installation. And further I want to keep the add ons on my website as small as possible. Therefore I have not taken this solution in account for my actual new Joomla 4 website.
But I repeat my statement from the other Thread here again:
I have an example from an older draft and study design of a website. There the function of the iframe in a modalbox is realized very good. If you open this site: idcreativ.de/index.php You can try the iframe / modalbox by clicking on the button "Hier geht's zum Text". You will experience, how responsive it is, on real desktop and also on real smartphone. You can change the smartphone from portrait to landscape and the modalbox will change its size adequately. Especially by the big top and bottom margins in portrait mode, the visitor will always recognize, that he is not visiting a regular website but the content of an article within a modalbox. Thus he can go on surfing only by closing the modal at first, instead of searching for a navbar.
BTW: This design has been created by the PLYR framework, a JS framework containing also media player and it is here: plyr.io/

on mobile it switches on a specific mode to use the whole screen area.

not acceptable on a real smartphone!

I don't understand what you mean here, because it has been developped for mobile
if you want, you can disable the mobile behavior in the plugin options

for the iframe option, the request is about to stop the scroll of the parent page, that you already asked. This is not possible for now, already added on my todolist

please download and unzip these files
www.dropbox.com/s/38vx0b83a0ejbbi/mediaboxck.zip?dl=0
and replace them in
plugins\system\mediabox_ck\assets

then clear your browser cache and tell me. It shall fix the scrolling of the body when the lightbox is opened
let me know

I have done, as you proposed. And I have left the code from an earlier proposal from you in the custom.css:
This in conjunction with your dropbox files does not do the trick. Sorry! You can see it on the webpage, I have already given you.

Of course I have then cleared the cache in Joomla and also the browser cache. Further I have tried it in Firefox and in Chromium.