Fix issue with Tailwind modifying global state

[remcohaszing]

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.

For reproduction, run the following script (Use native ESM):

import resolveConfig from "tailwindcss/lib/util/resolveConfig.js"
import typography from '@tailwindcss/typography'

const config = (color) => resolveConfig.default([{
  plugins: [typography],
  corePlugins: [],
  theme: {
    colors: {},
    extend: {
      typography: {
        DEFAULT: {
          css: {
            a: {
              color
            },
          },
        },
      },
    },
  },
}])

console.log("Config", false, JSON.stringify(config("#123456")).includes("#abcdef"))
console.log("Config", true, JSON.stringify(config("#abcdef")).includes("#abcdef"))
console.log("Config", false, JSON.stringify(config("#123456")).includes("#abcdef"))

As you can see, in the output, #abcdef is present in the third run, because the second run has added it to some internal state.

I have narrowed down the issue to these lines:

tailwindcss/src/util/resolveConfig.js

Lines 271 to 276 in db50bbb

	return normalizeConfig(
	defaults(
	{
	theme: resolveFunctionKeys(
	mergeExtensions(mergeThemes(allConfigs.map((t) => t?.theme ?? {})))
	),

I have refrained from writing a test yet, because although these changes fix it and all tests pass, it feels like it should be fixed somewhere else.

[thecrypticace]

@remcohaszing So defaults is meant to mutate the object passed in. What was happening is that, in recursive calls to mergeWith, we were overwriting the properties of an object. We always called it with a fresh object to merge into in every case except one: when it called itself recursively. Fixing that fixes the bug.