The tests pass on linux and the htest looks good.  Thank you for making all these changes.  It seems much clearer now, but I do want to spend more time reading the code to understand it.  Looks like you grouped all the font-related functions, so they are ready to become a Class?  The only part I couldn't figure out is how to update highlight_sample since it wouldn't be in the class.

With the backwards compatibility, I thought maybe you have an idea about how to make the changes compatible because of the issue that moves the extensions.  I can see where that would be a problem as far as making changes is concerned.

Maybe I'll look at the creating tests for one of the other tabs next?