I know you can do this with a filter, but a button would be lovely!
There is a dedicated Unread button in the folder list covering all folders.
If selecting the Filter button and then Unread is too much trouble, the other way to do this would be to create a search folder. Select the folder(s) for which you only want to view unread emails then Choose Fields and tick Marked as Unread, - this will add the Marked as Unread toggle switch to the main Search Folder window, where you can toggle it on. Clicking on this new Search Folder will then always display only unread emails for the folder(s) selected.
In the example below, I have selected the Inboxes for 4 separate email accounts and created a Unified view displaying unread emails only.
The other way is to make sure you have the Read Status column displayed and then to simply click the header to sort all emails by Read Status so that all unread emails are displayed together.
