LinkSuggest should use the jquery.textSelection API. If it works on the textarea directly, it can and will and is expected to break icw. CodeMirror and any other customised editor, like wikiEditor 2017, and various other syntax highlighting scripts.

This is not a CodeMirror issue.

I'm trying to change LinkSuggest to the textSelection API, but this alone doesn't solve the problem of LinkSuggest not working with CodeMirror.

LinkSuggest binds to the keydown and keypress events on the textarea to trigger the suggestions. However, when CodeMirror is active, the special editor doesn't forward those events to the native textarea, which means LinkSuggest doesn't detect any key events. I'm not sure what the possible solution would be without adding logic explicitly for CodeMirror, which looks a bit ugly to me, although CodeMirror itself has code to detect the presence of wikEd.