Hello, i noticed a problem when there is enough content on the page, when i click the modal popup link the background seems to move (and also when clicking the close button), i made a small sample video to see better the effect:
When opening the modal, the scrollbar is disabled on the underlying window.
How your website's css deals with the difference, is down to your templates css.
You can override the styling if you want to force back the scrollbars.
It's possible to have the close button inside the popup window and without the black overlay? (so remains all the page visible)
Like it was for Modals component in Joomla 3.x.