Fix issue with Tailwind modifying global state (#9294)

* Fix issue with Tailwind modifying global state

When running Tailwind, it modifies the plugin defaults parameters. As a
result Tailwind using a Tailwind plugin in the same process twice yields
different results.

* Add failing test

* Undo defaults change

* wip

* Fix shared mutation problem

* Update changelog

Co-authored-by: Jordan Pittman <jordan@cryptica.me>

CHANGELOG.md

@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Don't output duplicate utilities ([#9208](https://github.com/tailwindlabs/tailwindcss/pull/9208))
 - Fix `fontFamily` config TypeScript types ([#9214](https://github.com/tailwindlabs/tailwindcss/pull/9214))
 - Handle variants on complex selector utilities ([#9262](https://github.com/tailwindlabs/tailwindcss/pull/9262))
+- Don't mutate shared config objects ([#9294](https://github.com/tailwindlabs/tailwindcss/pull/9294))
 
 ## [3.1.8] - 2022-08-05
 

src/util/resolveConfig.js

@@ -29,7 +29,7 @@ function mergeWith(target, ...sources) {
 
       if (merged === undefined) {
         if (isObject(target[k]) && isObject(source[k])) {
-          target[k] = mergeWith(target[k], source[k], customizer)
+          target[k] = mergeWith({}, target[k], source[k], customizer)
         } else {
           target[k] = source[k]
         }

tests/resolveConfig.test.js

@@ -1763,3 +1763,30 @@ test('all helpers can be destructured from the first function argument', () => {
     },
   })
 })
+
+test('does not duplicate extended configs every time resolveConfig is called', () => {
+  let shared = {
+    foo: { bar: { baz: [{ color: 'red' }] } },
+  }
+
+  const createConfig = (color) =>
+    resolveConfig([
+      {
+        theme: {
+          foo: shared.foo,
+          extend: {
+            foo: { bar: { baz: { color } } },
+          },
+        },
+      },
+    ])
+
+  createConfig('orange')
+  createConfig('yellow')
+  createConfig('green')
+
+  const result = createConfig('blue')
+
+  expect(shared.foo.bar.baz).toMatchObject([{ color: 'red' }])
+  expect(result.theme.foo.bar.baz).toMatchObject([{ color: 'red' }, { color: 'blue' }])
+})