fix(runtime-dom): before update should also set css vars

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

@@ -385,6 +385,44 @@ describe('useCssVars', () => {
     }
   })
 
+  test('with delay mount child', async () => {
+    const state = reactive({ color: 'red' })
+    const value = ref(false)
+    const root = document.createElement('div')
+
+    const Child = {
+      setup() {
+        onMounted(() => {
+          const childEl = root.children[0]
+          expect(getComputedStyle(childEl!).getPropertyValue(`--color`)).toBe(
+            `red`,
+          )
+        })
+        return () => h('div', { id: 'childId' })
+      },
+    }
+    const App = {
+      setup() {
+        useCssVars(() => state)
+        return () => (value.value ? h(Child) : [h('span')])
+      },
+    }
+
+    render(h(App), root)
+    await nextTick()
+    // css vars use with fallback tree
+    for (const c of [].slice.call(root.children as any)) {
+      expect((c as HTMLElement).style.getPropertyValue(`--color`)).toBe(`red`)
+    }
+
+    // mount child
+    value.value = true
+    await nextTick()
+    for (const c of [].slice.call(root.children as any)) {
+      expect((c as HTMLElement).style.getPropertyValue(`--color`)).toBe(`red`)
+    }
+  })
+
   // #8826
   test('with custom element', async () => {
     const state = reactive({ color: 'red' })

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

@@ -3,8 +3,10 @@ import {
   Static,
   type VNode,
   getCurrentInstance,
+  onBeforeUpdate,
   onMounted,
   onUnmounted,
+  queuePostFlushCb,
   warn,
   watch,
 } from '@vue/runtime-core'
@@ -47,6 +49,12 @@ export function useCssVars(getter: (ctx: any) => Record<string, string>): void {
     updateTeleports(vars)
   }
 
+  // handle cases where child component root is affected
+  // and triggers reflow in onMounted
+  onBeforeUpdate(() => {
+    queuePostFlushCb(setVars)
+  })
+
   onMounted(() => {
     // run setVars synchronously here, but run as post-effect on changes
     watch(setVars, NOOP, { flush: 'post' })