Vivaldi loses focus when OS notifications show up.
I'm running Vivaldi on Elementary OS, which is an Ubuntu derivative. When the OS notifications show up (like when turning the volume up or down, or when the next song plays etc.), Vivaldi loses its focus, and I need to click inside the viewport again in order to be able to use page up / page down, arrows and tab.
For what it's worth, I don't have that issue with Ubuntu/Unity.