Function announce_scale_to_windows 

pub(crate) fn announce_scale_to_windows(
    state: &AppState,
    display_id: u32,
    output: &Output,
    scale: f64,
)

Tell every window on an output the output’s scale: the surface’s preferred buffer scale and fractional scale, so a client draws at the new density rather than being stretched to it. The size a client keeps is the policy’s business where one runs, and the fullscreen configure’s otherwise.