This is a weird one. By using intCast on the `idx` I am periodically
getting a panic on index out of bounds where the index is larger than
FontIndex can possibly be. Very strange!
I tried to just remove intCasts and believe it or not that worked.
Previously, `cat /dev/urandom` would trigger the issue in seconds and
now I've had it running 20+ minutes without the issue.
The additional `if` check is just a safety mechanism