1. Run the following:

yarn && yarn dev && yarn prod

2. Examine the two css files in dist/.

3. Note the difference in CSS order.

/* Dev Build*/

.ComponentC_main {
  color: black;
}

.ComponentB_main {
  color: lightblue;
}

/* Prod Build */

.ComponentB_main {
  color: lightblue;
}

.ComponentC_main {
  color: black;
}

UPDATE

This was fixed in v4.12.1 as described here. However, the issue broke again in the next minor release (v4.13.0). I pushed a branch (webpack-v4.13.0-bad) to demonstrate that:

git checkout webpack-v4.13.0-bad
yarn && yarn dev && yarn prod

And you will see that the css order differs between the dev and prod builds.

---

I can mitigate this by:

1. Setting sideEffects: true in package.json. I'd prefer to not do this.
   • To test: git checkout with-sideeffects and run the above steps

or

2. Don't transpile ESM imports to CJS (which I think prevents tree shaking? Although I could be wrong).
   • To test: git checkout no-esm and run the above.