Do not sort and format stylesheets that didn't change (#17824)

This PR improves the upgrade tooling at tiny bit to make sure that as
long as we didn't change any of the stylesheets, that we also don't sort
internal nodes and/or format the stylesheet at all.

This is important in case the Prettier rules are different or if a
totally different formatter is used.

Essentially, if we didn't have to change the stylesheets because of a
migration, we don't want to change it due to a formatter either.

Author: RobinMalfait

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - PostCSS: Ensure that errors in stylesheet dependencies are recoverable ([#17754](https://github.com/tailwindlabs/tailwindcss/pull/17754))
 - Upgrade: Correctly print variants starting with `@` ([#17814](https://github.com/tailwindlabs/tailwindcss/pull/17814))
 - Skip `color-mix(…)` when opacity is `100%` ([#17815](https://github.com/tailwindlabs/tailwindcss/pull/17815))
+- Upgrade: Don't format stylesheets when nothing changed ([#17824](https://github.com/tailwindlabs/tailwindcss/pull/17824))
 
 ## [4.1.4] - 2025-04-14
 

integrations/upgrade/index.test.ts

@@ -1266,7 +1266,6 @@ test(
 
       --- ./src/c.1.css ---
       @import './c.2.css' layer(utilities);
-
       .baz-from-c {
         color: green;
       }

packages/@tailwindcss-upgrade/src/index.ts

@@ -90,6 +90,7 @@ async function run() {
     let stylesheets = loadResults
       .filter((result) => result.status === 'fulfilled')
       .map((result) => result.value)
+    let originals = new Map(stylesheets.map((sheet) => [sheet, sheet.root.toString()]))
 
     // Analyze the stylesheets
     try {
@@ -213,6 +214,7 @@ async function run() {
 
     // Format nodes
     for (let sheet of stylesheets) {
+      if (originals.get(sheet) === sheet.root.toString()) continue
       await postcss([sortBuckets(), formatNodes()]).process(sheet.root!, { from: sheet.file! })
     }