Hi Elina,
I beleive this article might prove helpful, not sure if it's achievable with pure CSS (https://isotropic.co/how-to-show-an-element-after-scrolling-halfway-down-the-page-js/). Here you could set the scrollY check to be 20px. Just make sure to refer to the correct DOM element, as with most Mendix apps the window object is likely going to be fixed, and you need the layout element with vertical scroll. Here's a Stackoverflow solution with the same idea behind: https://stackoverflow.com/questions/72500421/css-how-to-hide-an-element-based-on-position-within-page
Hope this helps!
Best regards,
András