Function output_content_covers 

pub fn output_content_covers(
    space: &Space<Window>,
    display_id: u32,
    logical_w: f64,
    logical_h: f64,
) -> bool

Whether a client on this display has COMMITTED content spanning the whole output. Under forced fullscreen a window answers its configure at the output’s logical size, so one still carrying its pre-configure size reads as not covering and holds the display’s frames.