It's here, Ver. 1.0 of Vivaldi the browser for power users. The stacked tabs are handy.
http://thenextweb.com/apps/2016/04/06/vivaldis-browser/
Added:
Uses less memory too:
https://www.yahoo.com/tech/vivaldi-browser-hits-version-1-061540742.html
Been using Vivaldi daily since the first beta. It has replaced my need/desire for Opera 12.x. Still waiting for my 64-bit Linux update, but Windows seems to be working.
BGR did a write-up, just a little more than a fluff piece but it does cover some of the UI features
http://bgr.com/2016/04/08/vivaldi-vs-google-chrome-vs-microsoft-edge/