@snowpack/plugin-svelte can't import Svelte files from a pnpm monorepo

[antony]

Not sure whether this is one issue or two issues, but either way it's a blocker for Snowpack usage which has been plaguing me for some time, and seems to have become worse with the more recent minor release.

Fundamentally I have a simple monorepo with this layout:

monorepo_root/
├─ libraries/
│  ├─ a/
│  │  ├─ A.svelte
│  ├─ b/
│  │  ├─ B.svelte
├─ apps/
│  ├─ snowpack-app/
│  │  ├─ C.svelte
│  │  ├─ App.svelte

The repo is managed by rush, which uses pnpm.

If I import C into App, everything works fine.
If I import A or B into App, two things happen:

1. historically, despite logging a change to the console, Snowpack will never see changes to A or B, and only picks them up with snowpack dev --reload which makes snowpack usage basically pointless, since the dev cycle is just restart, restart, restart
2. As of more recently (Snowpack 2.18.x) I get the following error in the browser console:

The file it is failing to load is called A.svelte.proxy.js which differs to the app-local file C.svelte which it requests without the proxy.js extension.

Failed to load module script: The server responded with a non-JavaScript MIME type of "application/octet-stream". Strict MIME type checking is enforced for module scripts per HTML spec.

If I inspect the file it has been loaded with type application/octet-stream rather than the expected application/javascript. The contents are an uncompiled Svelte component. The way this differs from the component C.svelte which lives inside the snowpack app (i.e. is not a module in the monorepo) is that when C is loaded (which works fine), it:

1. does not have proxy in the URL: http://localhost:8080/C.js
2. Is a compiled svelte component
3. Is served with mime type application/javascript.

Not sure what's going on exactly so insights would be much appreciated.

I've included the repo for reproduction:

https://github.com/antony/rush-and-snowpack

Steps to reproduce:

npm i -g @microsoft/rush
git clone 
git@github.com:antony/rush-and-snowpack.git
rush update && rush rebuild && rush check
cd apps/snowpack-app
rushx start

Open your browser console and you can see the error.