Function focus_ring_reach 

pub fn focus_ring_reach(theme: &Theme) -> f64

How far past a control’s face its ink may reach under theme: the focus ring, the ground-coloured line that parts it from a fill of your colour, and their antialiasing. A 2 px mark stroked centred on a rect reaches no further, so a region drawn as a part of its own pads its bounds by this.