Shared/Font is hardly covered in unit tests (as opposed to Style/Font which is completely covered). And it presented some good opportunities for code optimization. I wrote and tested the new unit tests first, then optimized the code and confirmed that everything still works.
There is still a bit of a gap with "exact" measurements. I had tests ready, but had to withdraw them when I discovered they weren't quite portable (see https://github.com/php/php-src/issues/9073).
oleibman