Function backdrop 

pub fn backdrop(
    lines: &[Option<BannerLine<'_>>; 2],
    widths: [f32; 2],
) -> Option<Panel>

The panel the banner’s lines sit on, given each line’s measured width, or None while the banner is invisible.

It fades with the text: the panel’s alpha is the envelope the lines carry, so the backdrop never lingers after the words, or arrives before them.