The bug

Since 20w17a, Minecraft supports non-BMP characters (like emojis and some CJK characters) but most of them are still tofus in the game.

How to reproduce

1. Open the chat
2. Copy this character "𡂿" (it's a CJK-B character)
3. See a tofu but excepted a proper Chinese character

Possible Solutions

1. Include a .ttf file, and use it when the game can't find the characters in font images. May need to decide the font that is going to be included (like the SC and TC problems that I mentioned before in MC-180099; like readability problems that I met when using regular weight; like copyright problems). This solution might also fix the Arabic/Devengari/... ligature problems (when I try to fix the problem using Noto Sans the ligature looks fine but less blocky or pixel-ish)
2. Include more font images. That might not work if GNU Unifont is still missing these characters.