Function retain_host_frame 

pub(crate) fn retain_host_frame(
    host: &HostSession,
    cap: Option<&mut WlCapture>,
    display_id: u32,
    frame: HostFrame,
)

Keep frame as the display’s host content. Under a zero-copy session the frame it replaces goes back to the host only once no frame in flight to the encode thread reads it.