Function drives_screen 

pub(crate) fn drives_screen(nodes: &[OutputNode], node: &OutputNode) -> bool

Views share one output, so its clients are driven once a frame, by the fastest display capturing that screen (ties go to the lowest number) – a client asked to draw once per view would render as many times a frame as there are displays over it, one asked by nobody (the screen itself is often captured by views alone) would freeze on the frame it first painted, and one driven by a slower sibling would stream stale frames on the faster display.