I developed some plugins for Textadept some time ago. It's very lightweight and mature, but its dependence on different backends on different platforms introduc

I developed some plugins for Textadept some time ago. It's very lightweight and mature, but its dependence on different backends on different platforms introduced some frictions (QT vs. GTK etc.). We had a working Mermaid and image viewer plugin. It's really extensible. Never upstreamed them though. It uses Lua LPEG lexers, which are extremely easy way to de develop syntax parsers/LSPs for new or unsupported languages. Eventually our project moved to Pragtical, which has SDL as a cross-platform backend and uses the more modern meson as the build system, which made developing for it much easier.