I would mention stb_textedit.h, but I would not recommend it. It was an interesting thing to study. but the library has many shortcomings and is pain to integrate and use. It is used in ImGui, but somewhat modified. Just to illustrate the flaws - it can't be used with utf-8 bytes easily, there is a large switch on keyboard shortcuts to trigger actions so you have to fake keyboard input to script things, the default word boundary detection is pretty bad, and there is no way to nicely provide double or triple click selection.
The two notable functions are stb_text_locate_coord() and stb_textedit_find_charpos(), which connect the physical x,y coordinates with position in text buffer. They both iterate lines of text - accumulating y position; and the chars of last line - accumulating x position.
For windowing, drawing and OS integration, SDL with SDL_ttf is actually pretty good. SDL3_ttf API got an improvement and no longer requires zero-terminated strings so you don't need to relocate every chunk.
The copy in dear imgui sources has a few tweaks to make it possible to use with UTF-8 without unnecessary lookups: see `STB_TEXTEDIT_GETPREVCHARINDEX`,`STB_TEXTEDIT_GETNEXTCHARINDEX` and `stb_textedit_text()` use.
I generally love the STB libraries but I have to agree that stb_textedit is/was a false economy at least for my use case of Dear ImGui. I can't fault the design but if you want a fancy text editor the value you get from stb_textedit is rather minor compared to whole things, and it gets in the way. I will aim to remove it from dear imgui at some point.
The two notable functions are stb_text_locate_coord() and stb_textedit_find_charpos(), which connect the physical x,y coordinates with position in text buffer. They both iterate lines of text - accumulating y position; and the chars of last line - accumulating x position.
For windowing, drawing and OS integration, SDL with SDL_ttf is actually pretty good. SDL3_ttf API got an improvement and no longer requires zero-terminated strings so you don't need to relocate every chunk.