eepp VS lapce-cpp-clangd

No idea about your 1st q. But source is I guess at https://github.com/SpartanJ/eepp/tree/develop/src/tools/ecode

I currently support about 30-40% of the spec, but the important part is putting that to work nicely in your editor. I think that Lapce is supporting even less than that and their implementation is not very nice at the moment (they spawn every LSP server for each language at the start of the application, and, if you install many LSPs it's gonna eat a stupid amount of resources for no reason). If you're looking for a good implementation of the spec that it's similar to what you might need, take a look at the Kate implementation, they have done a great job (not only with the LSP!), and they cover a good chunk of the spec. And of course, you can take a look at my implementation, it does not depend on Qt and might be more readable in consequence.

Having said that, I'm working on an alternative GPU accelerated and retained-mode GUI called eepp (https://github.com/SpartanJ/eepp/) while I work on a new code editor similar to Zed called ecode (https://github.com/SpartanJ/ecode). The framework still needs a lot of work but presents some ideas that might be interesting to some. Retained-mode GUIs are a complex beast and it's hard to achieve simplicity (I think immediate-mode GUIs can't be beaten in that regard, but they have their own downsides). So I invite anyone interested in GPU accelerated GUIs to take a look at my projects (online demos are available), I'm looking for collaborators! (sadly C++ isn't very appealing these days, but my code-base is huge and I can't write everything from scratch in a new fancy and cool language).

For language support. You need to install plugins. In this case, you need to install the clangd plugin https://github.com/lapce-community/lapce-cpp-clangd However, it might still not work for you because of your weird file extension policy. AFAIK, the language detection is hardcoded although this can be fixed I think quite easily I think.