While composing this post the Lemmy web client went to lunch. This is the classic behaviour of Lemmy when it has a problem. No error, just infinite spinner. After experimentation, it turns out that it tries to be smart but fails when treating URLs written with the gemini:// scheme.

(edit) It’s probably trying to visit the link for that convenience feature of pre-filling the title. If it does not recognise the scheme, it should just accept it without trying to be fancy. It likely screws up on other schemes as well, like dict, ftp, news, etc.

The workaround is to embed the #Gemini link in the body of the post.