From fe15e02fb33f1aec0c82f51a2fa7135ea7534fdf Mon Sep 17 00:00:00 2001
From: Alex Snezhko <alexsnezhko89@gmail.com>
Date: Sat, 20 Sep 2025 18:18:01 -0700
Subject: [PATCH] fix(shared): class presence overrides

---
 .../__tests__/ssrRenderAttrs.spec.ts          |  6 ++
 .../shared/__tests__/normalizeProp.spec.ts    | 37 ++++++++++++
 packages/shared/src/normalizeProp.ts          | 60 +++++++++++++++----
 3 files changed, 93 insertions(+), 10 deletions(-)

diff --git a/packages/server-renderer/__tests__/ssrRenderAttrs.spec.ts b/packages/server-renderer/__tests__/ssrRenderAttrs.spec.ts
index 984387bb864..d7b69d4d93a 100644
--- a/packages/server-renderer/__tests__/ssrRenderAttrs.spec.ts
+++ b/packages/server-renderer/__tests__/ssrRenderAttrs.spec.ts
@@ -144,6 +144,12 @@ describe('ssr: renderClass', () => {
     expect(ssrRenderClass(`"><script`)).toBe(`&quot;&gt;&lt;script`)
   })
 
+  test('overrides + duplicates', () => {
+    expect(
+      ssrRenderClass(['foo foo', { foo: true }, 'bar', { bar: false }]),
+    ).toBe('foo foo foo')
+  })
+
   test('className', () => {
     expect(
       ssrRenderAttrs({
diff --git a/packages/shared/__tests__/normalizeProp.spec.ts b/packages/shared/__tests__/normalizeProp.spec.ts
index 00c6b294da1..c1efdc23026 100644
--- a/packages/shared/__tests__/normalizeProp.spec.ts
+++ b/packages/shared/__tests__/normalizeProp.spec.ts
@@ -76,6 +76,43 @@ describe('normalizeClass', () => {
     ).toEqual('foo bar baz qux quux')
   })
 
+  test('handles overrides correctly', () => {
+    expect(
+      normalizeClass([
+        { foo: true },
+
+        'bar',
+        { bar: false, foo: undefined },
+
+        ' baz ',
+        { baz: false },
+
+        { qux: false },
+        ' qux \n  baz  ',
+
+        { quux: true },
+        [' quux quux2 ', [{ ' quux  quux3 ': false }]],
+
+        'quux3',
+      ]),
+    ).toEqual('qux baz quux2 quux3')
+  })
+
+  test('handles duplicates correctly', () => {
+    expect(
+      normalizeClass([
+        'foo  foo  baz  baz  baz',
+        { foo: true },
+
+        'bar bar ',
+        { bar: false },
+
+        { baz: false },
+        'baz baz',
+      ]),
+    ).toEqual('foo foo foo baz baz')
+  })
+
   // #6777
   test('parse multi-line inline style', () => {
     expect(
diff --git a/packages/shared/src/normalizeProp.ts b/packages/shared/src/normalizeProp.ts
index ef598a03ced..dc9ba856dc8 100644
--- a/packages/shared/src/normalizeProp.ts
+++ b/packages/shared/src/normalizeProp.ts
@@ -61,24 +61,64 @@ export function stringifyStyle(
 }
 
 export function normalizeClass(value: unknown): string {
-  let res = ''
+  // do not implicitly deduplicate classes if user duplicates same class
+  const classes: (string | undefined)[] = []
+  // keep track of where in the above array each class name is specified
+  // to allow e.g. `<p class="my-class" :class="{ 'my-class': false }">`
+  // to remove `my-class` from the element
+  const classIndexes = new Map<string, number[]>()
+
+  normalizeClassHelper(value, classes, classIndexes)
+
+  let str = ''
+  for (const klass of classes) {
+    if (klass) {
+      str += klass + ' '
+    }
+  }
+  return str.trim()
+}
+
+function normalizeClassHelper(
+  value: unknown,
+  classes: (string | undefined)[],
+  classIndexes: Map<string, number[]>,
+): void {
   if (isString(value)) {
-    res = value
+    const splitClasses = value.trim().split(/\s+/)
+    for (const klass of splitClasses) {
+      if (!classIndexes.has(klass)) {
+        classIndexes.set(klass, [])
+      }
+      classIndexes.get(klass)!.push(classes.length)
+      classes.push(klass)
+    }
   } else if (isArray(value)) {
     for (let i = 0; i < value.length; i++) {
-      const normalized = normalizeClass(value[i])
-      if (normalized) {
-        res += normalized + ' '
-      }
+      normalizeClassHelper(value[i], classes, classIndexes)
     }
   } else if (isObject(value)) {
-    for (const name in value) {
-      if (value[name]) {
-        res += name + ' '
+    for (const names in value) {
+      const splitClasses = names.trim().split(/\s+/)
+      for (const klass of splitClasses) {
+        if (value[names]) {
+          if (!classIndexes.has(klass)) {
+            classIndexes.set(klass, [])
+          }
+          classIndexes.get(klass)!.push(classes.length)
+          classes.push(klass)
+        } else {
+          const indexes = classIndexes.get(klass)
+          if (indexes) {
+            for (const i of indexes) {
+              classes[i] = undefined
+            }
+            classIndexes.set(klass, [])
+          }
+        }
       }
     }
   }
-  return res.trim()
 }
 
 export function normalizeProps(