Function hypot_fixed 

fn hypot_fixed(a: Fixed, b: Fixed) -> Fixed

Approximate magnitude of a transform basis vector.

According to FreeType this algorithm is a “guess” that works better than the one Apple documents, so both fixed point modes use it to scale a component offset that asks to be scaled.

https://github.com/freetype/freetype/blob/b1c90733ee6a04882b133101d61b12e352eeb290/src/truetype/ttgload.c#L1259