Before the recent change in Xfinity-e-mail, if I'd opened an e-mai, read it and wanted to show it as new, I'd place my arrow on it, and press the right side of the cursor. It would then show it as new (unopened).
It doesn't work that way now, and I don't know how to use that feature. H-E-L-P !!!
Select the checkbox to the left of the message. Then up in the toolbar over to the right click on the 3 horizontal lines to access more actions. In that menu select mark as unread.