Vivaldi is being built by former Opera developers, so you'll notice a lot of similarities like mouse gestures and the sidebar on the left (with a promised Mail

Vivaldi is being built by former Opera developers, so you'll notice a lot of similarities like mouse gestures and the sidebar on the left (with a promised Mail client coming also). I'm actually surprised that the "new" Opera hasn't tried to build in many of the features that made the "old" Opera (12.x) so popular among its small fan base.