fix(sfc/style-vars): should attach css vars while subtree changed (#2178)

* fix(cssVars): should attach css vars while `subtree` changed

fix #2177

* fix: fix test

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

@@ -1,4 +1,5 @@
 import {
+  ref,
   render,
   useCssVars,
   h,
@@ -125,4 +126,29 @@ describe('useCssVars', () => {
       id
     )
   })
+
+  test('with subTree changed', async () => {
+    const state = reactive({ color: 'red' })
+    const value = ref(true)
+    const root = document.createElement('div')
+
+    const App = {
+      setup() {
+        useCssVars(() => state)
+        return () => (value.value ? [h('div')] : [h('div'), h('div')])
+      }
+    }
+
+    render(h(App), root)
+    // 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`)
+    }
+
+    value.value = false
+    await nextTick()
+    for (const c of [].slice.call(root.children as any)) {
+      expect((c as HTMLElement).style.getPropertyValue(`--color`)).toBe('red')
+    }
+  })
 })