feat(custom-element): support css `:host` selector by applying css vars on host element (#8830)

close #8826

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

@@ -4,6 +4,7 @@ import {
   Suspense,
   Teleport,
   createStaticVNode,
+  defineCustomElement,
   h,
   nextTick,
   reactive,
@@ -381,4 +382,26 @@ describe('useCssVars', () => {
       expect((c as HTMLElement).style.getPropertyValue(`--color`)).toBe('red')
     }
   })
+
+  // #8826
+  test('with custom element', async () => {
+    const state = reactive({ color: 'red' })
+    const container = document.createElement('div')
+    const App = defineCustomElement({
+      styles: [`div { color: red; }`],
+      setup() {
+        useCssVars(() => state)
+        return () => {
+          return h('div', 'hello')
+        }
+      },
+    })
+    customElements.define('css-vars-ce', App)
+    container.innerHTML = `<css-vars-ce></css-vars-ce>`
+    document.body.appendChild(container)
+    await nextTick()
+    expect(container.innerHTML).toBe(
+      `<css-vars-ce style="--color: red;"></css-vars-ce>`,
+    )
+  })
 })

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

@@ -38,7 +38,11 @@ export function useCssVars(getter: (ctx: any) => Record<string, string>) {
 
   const setVars = () => {
     const vars = getter(instance.proxy)
-    setVarsOnVNode(instance.subTree, vars)
+    if (instance.ce) {
+      setVarsOnNode(instance.ce as any, vars)
+    } else {
+      setVarsOnVNode(instance.subTree, vars)
+    }
     updateTeleports(vars)
   }