Function separator_rect 

pub fn separator_rect(x0: f64, x1: f64, y: f64, inset: f64) -> Option<Rect>

The one device-pixel row a separator centred on y occupies: y is snapped so a caller passing a whole-pixel coordinate gets a crisp rule, not a two-row smear at half alpha (which would fall under HAIRLINE_MIN_DELTA even though the colour itself registers).