Agree, please add this ability
This already exists. Right click the folder and select Mark All as Read or (what I do) put the Mark All as Read function on the Quick Access Toolbar.
Currently 'Mark all emails as read' will mark both focused and other emails as read, even though on the screen I only see one or other set. This is not good behaviour. If I am looking at my unread focused emails and choose to mark all unread ones as read then I should be able to independently go to my Other emails and review the unread emails then mark these as read if I wish.