I am proposing possible adjustments to your CC support, taking advantage of libass. These can be made into options, selectable at the user's discretion.
- First, you still currently use a proportional font for the CC subtitles. Since CC in their raw form use a fixed-width font, it might be useful to have this as an option, perhaps using font scaling parameters so that the position of the subtitle would better match that as seen on an actual TV. If necessary, one can attach a free-to-use fixed-width font (like Deja Vu Sans Mono) to an MKV file and have the subtitle track refer to it.
- Another option is to use an alternate subtitle style. Your style is more typical for subtitles, but the ASS style set allows for using a solid-color background (in this case, black) for the subtitles, which is again closer to how CC is displayed on a real TV.
- One last request, a minor one and not related to libass. Matroska files allow not just the audio but also the subtitle tracks to have names. You might wish to allow the option to name subtitle tracks. In particular, even if you don't expand this as a user option, it might be convenient to use this ability to insert a "Closed Captions" label on CC subtitle tracks placed in MKV files.