Fix sorting numeric utilities when they have different magnitudes (#16414)

When sorting utilities it was possible for a utility like `duration-700`
to come before `duration-75` or `duration-1000` to come before
`duration-150`.

This PR fixes this behavior by reading the full "magnitude" of each
numbers before we compare them.

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Export `tailwindcss/lib/util/flattenColorPalette.js` for backward compatibility ([#16411](https://github.com/tailwindlabs/tailwindcss/pull/16411))
+- Fix sorting numeric utilities when they have different magnitudes ([#16414](https://github.com/tailwindlabs/tailwindcss/pull/16414))
 
 ## [4.0.6] - 2025-02-10
 

packages/tailwindcss/src/utils/compare.test.ts

@@ -21,6 +21,12 @@ it.each([
   ['2', '1', GREATER],
   ['1', '10', LESS],
   ['10', '1', GREATER],
+
+  // Numbers of different lengths
+  ['75', '700', LESS],
+  ['700', '75', GREATER],
+  ['75', '770', LESS],
+  ['770', '75', GREATER],
 ])('should compare "%s" with "%s" as "%d"', (a, b, expected) => {
   expect(Math.sign(compare(a, b))).toBe(expected)
 })
@@ -124,3 +130,39 @@ it('should sort strings with multiple numbers consistently using the `compare` f
     ]
   `)
 })
+
+it('sort is stable', () => {
+  // Heap's algorithm for permutations
+  function* permutations<T>(input: T[]) {
+    let pos = 1
+    let stack = input.map(() => 0)
+
+    yield input.slice()
+
+    while (pos < input.length) {
+      if (stack[pos] < pos) {
+        let k = pos % 2 == 0 ? 0 : stack[pos]
+        ;[input[k], input[pos]] = [input[pos], input[k]]
+        yield input.slice()
+        ++stack[pos]
+        pos = 1
+      } else {
+        stack[pos] = 0
+        ++pos
+      }
+    }
+  }
+
+  let classes = ['duration-initial', 'duration-75', 'duration-150', 'duration-700', 'duration-1000']
+
+  for (let permutation of permutations(classes)) {
+    let sorted = [...permutation].sort(compare)
+    expect(sorted).toEqual([
+      'duration-75',
+      'duration-150',
+      'duration-700',
+      'duration-1000',
+      'duration-initial',
+    ])
+  }
+})

packages/tailwindcss/src/utils/compare.ts

@@ -14,9 +14,6 @@ export function compare(a: string, z: string) {
     let aCode = a.charCodeAt(i)
     let zCode = z.charCodeAt(i)
 
-    // Continue if the characters are the same
-    if (aCode === zCode) continue
-
     // If both are numbers, compare them as numbers instead of strings.
     if (aCode >= ZERO && aCode <= NINE && zCode >= ZERO && zCode <= NINE) {
       let aStart = i
@@ -35,14 +32,24 @@ export function compare(a: string, z: string) {
       let aNumber = a.slice(aStart, aEnd)
       let zNumber = z.slice(zStart, zEnd)
 
-      return (
-        Number(aNumber) - Number(zNumber) ||
-        // Fallback case if numbers are the same but the string representation
-        // is not. Fallback to string sorting. E.g.: `0123` vs `123`
-        (aNumber < zNumber ? -1 : 1)
-      )
+      let diff = Number(aNumber) - Number(zNumber)
+      if (diff) return diff
+
+      // Fallback case if numbers are the same but the string representation
+      // is not. Fallback to string sorting. E.g.: `0123` vs `123`
+      if (aNumber < zNumber) return -1
+      if (aNumber > zNumber) return 1
+
+      // Continue with the next character otherwise short strings will appear
+      // after long ones when containing numbers. E.g.:
+      // - bg-red-500/70
+      // - bg-red-500
+      continue
     }
 
+    // Continue if the characters are the same
+    if (aCode === zCode) continue
+
     // Otherwise, compare them as strings
     return aCode - zCode
   }