When you enable the tabbed view inside Evernote, it would be more useful for the tabs to display the name of the specific 'note' you're working on, rather than the name of the Notebook. This is especially useful where you're working on multiple notes within the same notebook (that you've split across various tabs). As in the example below, you can see that both tabs display "All Notes" and "Diary" however its the same case if I was working on two different notes in say the same notebook "Diary"
This can get a bit confusing, because if they both display the same name, you have to then click on each of the tabs to find out which one it was that you were working on.
My recommendation is to essentially display the name of the Note, or you can show the path of the note i.e. Diary > 12102017 just to make this feature that bit more useful (just like how we currently have it when you open a note in a new window). The tabbed view becomes less useful if you have a number of notebooks and notes inside them that you might be working on at the same time and therefore harder to manage.
I've also attached a screenshot of what the "open in a new window" looks like.