Constant FONT_METRIC_FUDGE_FACTOR 

const FONT_METRIC_FUDGE_FACTOR: f64 = 1.5_f64;

When the ascent or descent font metrics for a text string are NaN, we fall back on the actual metrics (instead of font metrics). But the actual metrics don’t include the inter-line spacing, so we use a fudge factor to guess at something appropriate.