I think the default behavior is to only re-render if the window is active/focused. You can trigger a render at specific points, including in the main loop, which will result in the behavior you mention.

This can be problematic, e.g. some of the sensor interfaces I have, I want to always display correct data, even if not focused. So, I have to decide if I want to have old data shown in the background misleading users, or have a per penalty from constant renders. Or try something else to be clever. (Maybe have it update at a low rate if not focused? I think that's the move...)