fix(css-vars): nullish v-bind in style should not lead to unexpected inheritance

packages/compiler-sfc/__tests__/__snapshots__/compileScript.spec.ts.snap

@@ -884,9 +884,9 @@ export default {
 
 return (_ctx, _push, _parent, _attrs) => {
   const _cssVars = { style: {
-  "--xxxxxxxx-count": (count.value),
-  "--xxxxxxxx-style\\\\.color": (style.color),
-  "--xxxxxxxx-height\\\\ \\\\+\\\\ \\\\\\"px\\\\\\"": (height.value + "px")
+  ":--xxxxxxxx-count": (count.value),
+  ":--xxxxxxxx-style\\\\.color": (style.color),
+  ":--xxxxxxxx-height\\\\ \\\\+\\\\ \\\\\\"px\\\\\\"": (height.value + "px")
 }}
   _push(\`<!--[--><div\${
     _ssrRenderAttrs(_cssVars)

packages/compiler-sfc/__tests__/compileScript.spec.ts

@@ -646,10 +646,10 @@ describe('SFC compile <script setup>', () => {
       expect(content).toMatch(`return (_ctx, _push`)
       expect(content).toMatch(`ssrInterpolate`)
       expect(content).not.toMatch(`useCssVars`)
-      expect(content).toMatch(`"--${mockId}-count": (count.value)`)
-      expect(content).toMatch(`"--${mockId}-style\\\\.color": (style.color)`)
+      expect(content).toMatch(`":--${mockId}-count": (count.value)`)
+      expect(content).toMatch(`":--${mockId}-style\\\\.color": (style.color)`)
       expect(content).toMatch(
-        `"--${mockId}-height\\\\ \\\\+\\\\ \\\\\\"px\\\\\\"": (height.value + "px")`,
+        `":--${mockId}-height\\\\ \\\\+\\\\ \\\\\\"px\\\\\\"": (height.value + "px")`,
       )
       assertCode(content)
     })

packages/compiler-sfc/src/style/cssVars.ts

@@ -23,7 +23,12 @@ export function genCssVarsFromList(
   return `{\n  ${vars
     .map(
       key =>
-        `"${isSSR ? `--` : ``}${genVarName(id, key, isProd, isSSR)}": (${key})`,
+        // The `:` prefix here is used in `ssrRenderStyle` to distinguish whether
+        // a custom property comes from `ssrCssVars`. If it does, we need to reset
+        // its value to `initial` on the component instance to avoid unintentionally
+        // inheriting the same property value from a different instance of the same
+        // component in the outer scope.
+        `"${isSSR ? `:--` : ``}${genVarName(id, key, isProd, isSSR)}": (${key})`,
     )
     .join(',\n  ')}\n}`
 }

packages/runtime-core/src/component.ts

@@ -582,13 +582,13 @@ export interface ComponentInternalInstance {
    * For updating css vars on contained teleports
    * @internal
    */
-  ut?: (vars?: Record<string, string>) => void
+  ut?: (vars?: Record<string, unknown>) => void
 
   /**
    * dev only. For style v-bind hydration mismatch checks
    * @internal
    */
-  getCssVars?: () => Record<string, string>
+  getCssVars?: () => Record<string, unknown>
 
   /**
    * v2 compat only, for caching mutated $options

packages/runtime-core/src/hydration.ts

@@ -938,10 +938,8 @@ function resolveCssVars(
   ) {
     const cssVars = instance.getCssVars()
     for (const key in cssVars) {
-      expectedMap.set(
-        `--${getEscapedCssVarName(key, false)}`,
-        String(cssVars[key]),
-      )
+      const value = String(cssVars[key] ?? 'initial')
+      expectedMap.set(`--${getEscapedCssVarName(key, false)}`, value)
     }
   }
   if (vnode === root && instance.parent) {

packages/runtime-dom/__tests__/helpers/useCssVars.spec.ts

@@ -465,4 +465,23 @@ describe('useCssVars', () => {
     render(h(App), root)
     expect(colorInOnMount).toBe(`red`)
   })
+
+  test('should set vars as `initial` for nullish values', async () => {
+    const state = reactive<Record<string, unknown>>({
+      color: undefined,
+      size: null,
+    })
+    const root = document.createElement('div')
+    const App = {
+      setup() {
+        useCssVars(() => state)
+        return () => h('div')
+      },
+    }
+    render(h(App), root)
+    await nextTick()
+    const style = (root.children[0] as HTMLElement).style
+    expect(style.getPropertyValue(`--color`)).toBe('initial')
+    expect(style.getPropertyValue(`--size`)).toBe('initial')
+  })
 })

packages/runtime-dom/src/helpers/useCssVars.ts

@@ -17,7 +17,9 @@ export const CSS_VAR_TEXT: unique symbol = Symbol(__DEV__ ? 'CSS_VAR_TEXT' : '')
  * Runtime helper for SFC's CSS variable injection feature.
  * @private
  */
-export function useCssVars(getter: (ctx: any) => Record<string, string>): void {
+export function useCssVars(
+  getter: (ctx: any) => Record<string, unknown>,
+): void {
   if (!__BROWSER__ && !__TEST__) return
 
   const instance = getCurrentInstance()
@@ -64,7 +66,7 @@ export function useCssVars(getter: (ctx: any) => Record<string, string>): void {
   })
 }
 
-function setVarsOnVNode(vnode: VNode, vars: Record<string, string>) {
+function setVarsOnVNode(vnode: VNode, vars: Record<string, unknown>) {
   if (__FEATURE_SUSPENSE__ && vnode.shapeFlag & ShapeFlags.SUSPENSE) {
     const suspense = vnode.suspense!
     vnode = suspense.activeBranch!
@@ -94,13 +96,14 @@ function setVarsOnVNode(vnode: VNode, vars: Record<string, string>) {
   }
 }
 
-function setVarsOnNode(el: Node, vars: Record<string, string>) {
+function setVarsOnNode(el: Node, vars: Record<string, unknown>) {
   if (el.nodeType === 1) {
     const style = (el as HTMLElement).style
     let cssText = ''
     for (const key in vars) {
-      style.setProperty(`--${key}`, vars[key])
-      cssText += `--${key}: ${vars[key]};`
+      const value = String(vars[key] ?? 'initial')
+      style.setProperty(`--${key}`, value)
+      cssText += `--${key}: ${value};`
     }
     ;(style as any)[CSS_VAR_TEXT] = cssText
   }

packages/server-renderer/__tests__/ssrRenderAttrs.spec.ts

@@ -203,4 +203,15 @@ describe('ssr: renderStyle', () => {
       }),
     ).toBe(`color:"><script;`)
   })
+
+  test('useCssVars handling', () => {
+    expect(
+      ssrRenderStyle({
+        fontSize: null,
+        ':--color': undefined,
+        ':--size': null,
+        '--foo': 1,
+      }),
+    ).toBe(`--color:initial;--size:initial;--foo:1;`)
+  })
 })

packages/server-renderer/src/helpers/ssrRenderAttrs.ts

@@ -1,5 +1,7 @@
 import {
   escapeHtml,
+  isArray,
+  isObject,
   isRenderableAttrValue,
   isSVGTag,
   stringifyStyle,
@@ -93,6 +95,22 @@ export function ssrRenderStyle(raw: unknown): string {
   if (isString(raw)) {
     return escapeHtml(raw)
   }
-  const styles = normalizeStyle(raw)
+  const styles = normalizeStyle(ssrResetCssVars(raw))
   return escapeHtml(stringifyStyle(styles))
 }
+
+function ssrResetCssVars(raw: unknown) {
+  if (!isArray(raw) && isObject(raw)) {
+    const res: Record<string, unknown> = {}
+    for (const key in raw) {
+      // `:` prefixed keys are coming from `ssrCssVars`
+      if (key.startsWith(':--')) {
+        res[key.slice(1)] = raw[key] ?? 'initial'
+      } else {
+        res[key] = raw[key]
+      }
+    }
+    return res
+  }
+  return raw
+}