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After a few tries I managed to reproduce satisfying results with this add-on, but noticed Gmail is VERY particular about how you go about this—if you are already on https://mail.google.com/intl/en/mail/help/about.html when you install this add-on (or attempt to access Gmail through that URL after installation), you will NOT be redirected to https://mail.google.com/. You MUST begin at https://mail.google.com/ (or the aliased URL https://www.gmail.com) for this to work.
@developer: Love your add-ons!
Followed your steps to a t and I get no redirect.
Oh! You are lucky!
It is 100% reproducible in my environment.