Can't use esc key to exit fullscreen video on Prime
I'm having the exact same problem using Amazon Prime video. Escape key does nothing, and I have to manually (mouse) bring the video out of full screen mode. Esc works fine on one of my other computers. Both machines are using Ubuntu 14.04, and are fully up to date.
Yes, a global (known) bug in Vivaldi where Esc key does not leave fullscreen.