Illumination of objects outside the viewport also affects the objects inside, but no problem. Imagine that you select a small area into the viewport and click on “Build region” feature: objects into the region would be rendered and affected by the illumination of the entire scene. And the render will disappear if you move the viewport or the cam.
Vray can do this (render region) and calculates the illumination of the entire scene in a small window. It is perfect to testing without long time waiting.
I think this is highly necessary to work with static lighting because currently we have to wait even 3 days to check if the result of light build is right or wrong … so I beg Epic to add some feature that makes it easier to work with static lighting.