Should I be able to do this in a custom HTML page :
<script src="https://cdnjs.cloudflare.com/ajax/libs/codemirror/5.28.0/mode/javascript/javascript.min.js"></script>
If I copy the JS into a <script> block it works fine, but it doesn’t seem to find the JS if I include it like this.
It is entirely possible I’m doing something daft, but a nod to whether there is some internal block or not would be great.
I fully appreciate I’m going a little off piste here, and accept that everything I’m doing may not work 
Ha, yes - this doesn’t work because of a quirk in a library we use. (You’re right that you’re a little off-piste here, and including <script> tags in HTML templates like this isn’t supported behaviour 
)
However, we will shortly be shipping an expanded theming system that will support hi-jinks like this. Could I suggest that you make do with a TextArea with a fixed-width font for the next few weeks, or is this a critical feature?
I like to push things a little
Nah, it’s not critical. Nice to have and will allow me to integrate something I didn’t think I’d be able to, but definitely not essential.
To be clear, being able to integrate something like CodeMirror (and then publishing a “CodeMirror component” for Anvil as a reusable unit) is definitely on our roadmap. This is just a matter of how to get there