Earlier quoted context omitted.
If it was a solved problem in the 90s, can't you just use a text editor from the 90s? No need to bash on this project just because it doesn't fulfill your use case.
Because there's contemporary Markdown editors that do work as expected. I didn't bash this one, I asked.
Modern editors are fairly complex. Splitting a bitbuffer and showing ascii line by line is simple. But modern editors have to deal with unicode, where some characters (surrogates) are two chars long, and can be followed by modifiers such as skin color of emojis. Then some characters are wider then others. So a modern editor must first parse the string encoding, then parse the language (markdown) for further formatting and coloring or building a WYSIWYG, usually on every key stroke. The expensive part is showing the text on the screen, generating the fonts, where old editors didnt have fonts.