If you happen to have a long registration form (due to all optional fields being visible), here’s some CSS to add a scrollbar to the window:

div#doc-layout.doc-layout.doc-layout-external-ecommerce .ui-dialog-content.ui-dialog-size-lg.is-visible {
    display: inline-block;
    height: 90%;
    overflow-y: auto;
    overflow-x: hidden;
}

As a more user-friendly solution, I’ve opened an idea to suppress optional user fields from the registration window altogether (but still make them available for users to fill out later).  Please upvote: