Interesting, I actually expected it to encode a single letter with infinitely long combining marks such that 'highlighting' it was just highlighting one character.

You can do that too, if you increase the STACK_HEIGHT constant (btw, the decoder still works the same, so changing this doesn't break compatibility)

Oh neat! Thanks :)

Most of the characters appear as boxes on my phone.

That's curious, because the only character is just the letter A. But I suppose if the font doesn't support a particular combining mark, it gives up on the whole grapheme?