Widget auto-scroll pushes content behind floating headers

The buyback widget auto-scrolls to align with the top of the page when an element is clicked, but our site (and many others) has a floating header for the menu, search bar etc.

We need the ability to either stop this behaviour entirely, specify an offset amount to the scroll (which would need to be adaptive based on screen size as the header on mobile is different to the desktop version), or we need direct control over the CSS for that element. I’ve tried every workaround I can think of but nothing will prevent the scroll action from losing content behind the header.