Using React components inside a meteor package in a meteor 3 project will result in a React warning about multiple instances of React in the same app. It's a regression since it works fine with meteor 2.

How to reproduce

• clone this repo

Test with meteor 2 (working as expected)

• open a terminal and run the following commands:

./bin/run2

• open a browser and go to http://localhost:3000/

• you should see a grey line where you can click and enter some text

• open the developer console and you should see nothing

Test with meteor 3 (not working)

• open a terminal and run the following commands:

./bin/run3

• open a browser and go to http://localhost:3000/

• you should not see the grey line

• open the developer console and you should see the following output:

Warning: Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for one of the following reasons:
1. You might have mismatching versions of React and the renderer (such as React DOM)
2. You might be breaking the Rules of Hooks
3. You might have more than one copy of React in the same app
See https://reactjs.org/link/invalid-hook-call for tips about how to debug and fix this problem.

You'll see that there's 2 instances of react-dom in the projet if you run the following command:

❯ find . -name react-dom
./.meteor/local/isopacks/sakod_core/npm/node_modules/react-dom
./.meteor/local/build/programs/server/npm/node_modules/meteor/sakod_core/node_modules/react-dom
./.meteor/local/build/programs/server/npm/node_modules/react-dom

--> ./node_modules/react-dom
--> ./packages/sakod-core/.npm/package/node_modules/react-dom

One in the node_modules and one in the meteor package and it explains the React warning (You might have more than one copy of React in the same app).

Doing the same test with meteor 2 will work as expected and find . -name react-dom will only return the node_modules instance.

I really have no idea why this is happening, is it a changes in the meteor building system or in npm?