Function paint_icon_button 

pub fn paint_icon_button(
    scene: &mut Scene,
    transform: Affine,
    text: &mut TextRenderer,
    theme: &Theme,
    _: (Rect, Icon),
    face: ButtonFace,
)

Paint one icon button of a window’s bar with the shared button face (crate::widget::Button::paint): the app’s slots (crate::window_toolbar), the History toggle and the new-tab button are the same control, so everything the app provides in the bar stands on the window’s ground as one row of them — the window’s own controls are the pill’s, not these.