Before SharePoint Online, I embedded cross-origin iframes to applications using the method described here. It uses a basic JavaScript Event Listener to receive requests from the child, such as resizing an iframe object on the parent to match the height of the content within. I modified this code to also use contain an Event Listener to scroll to the top of the parent when a page loads within the iframe. But this recently broke when Microsoft removed some classes from the scrollable div of Modern SharePoint Online pages. But no worries, I have a vanilla JavaScript solution.