Hi twosevenths ,
Thanks for reaching out to us!
The black part of the editor is due to the limitations of our new way of showing a preview of the site.
We now create a screenshot of the page of the site to show it in the preview but the screenshots are limited by how wide we can make them. The screenshots also don't show the characters well.
The focus of the preview is to see how the button will generally look like on your site and to see the position of the button on the page.
I hope this answers the question as to why the preview looks different after the update.
If you have any questions, please let me know! 😄