Constant EDGE_ALPHA_STEPS 

pub const EDGE_ALPHA_STEPS: &'static [u8];

Alphas edge_on tries, ascending, until the hairline visibly moves the surface it is drawn on (HIG §9.5). Step 0 is the control hairline (chrome::shell::HAIRLINE_ALPHA); the ladder only climbs on grounds so dark or so light that a 26-alpha wash lands within rounding distance of the surface — relative luminance compresses at the dark end, so pure black needs more alpha for the same visible step.