Random full screen
I am running the latest version of Vivaldi on my Macbook Air (Siera OS10.12.4) and it will randomly go into Full Screen! There is nothing I can do to get it out of Full Screen but then just as suddenly it will right itself, weird and rather disconcerting. Any ideas what to do when this happens to right things?
You can open mission control with the shortcut -- normally F3. From there you can hover over the window which is in fullscreen, and go out of fullscreen by clicking the button that appears.
Thank you @luetage for this, that works but presumably it is a bug that needs fixing.