When I click on attachments to preview them, they appear through overlay that has to be clicked out of instead of appearing beside the email message as it was in the old yahoo mail.

Being able to view them beside my email message made cross-checking and referencing information from the attachments much easier instead of having to close and open the preview each time.

This would also be a more efficient use of space on for desktop users who have ample horizontal display space for such a layout.