Function corner_button 

pub(crate) fn corner_button(
    ui: &mut Ui,
    id: &str,
    top: f32,
    label: RichText,
) -> bool

Draw a button that owns a corner, and report whether it was pressed.

id names the area, so the button can be read back — the docs link is placed under the fullscreen button by measuring it — and so two corners can be tested apart. A free function rather than the body of a panel’s closure so a test can drive it against a bare [egui::Context].