Function card_width 

pub fn card_width(window: Rect, max_width: f64, theme: &Theme) -> f64

Width of the sheet for max_width (design pixels) inside window, knowable before any body row is measured. Widths grow with the text scale, because what fills a sheet is text. A floating sheet keeps the content gutter clear of every window edge; a compact page fills the window and keeps that gutter inside its body.