Scroll Groups not working with Fixed position
Steps to reproduce:
1. Create an artboard taller than the viewport height (simulating a vertical scrolling mobile experience)
2. Create a group intended for horizontal scrolling within the viewport area
3. Apply the Horizontal Scroll option in the sidebar (optional: preview to ensure it's working correctly)
4. Check the box for Fix Position When Scrolling
As soon as I check the "Fix position when scrolling" option the horizontal scroll won't work anymore.

This has been reported here but has never been solved.
