fix(custom-element): ensure child component styles are injected in correct order before parent styles

packages/runtime-core/src/component.ts

@@ -1256,7 +1256,7 @@ export interface ComponentCustomElementInterface {
   /**
    * @internal
    */
-  _injectChildStyle(type: ConcreteComponent): void
+  _injectChildStyle(type: ConcreteComponent, parent?: ConcreteComponent): void
   /**
    * @internal
    */

packages/runtime-core/src/renderer.ts

@@ -1349,7 +1349,8 @@ function baseCreateRenderer(
         } else {
           // custom element style injection
           if (root.ce) {
-            root.ce._injectChildStyle(type)
+            const parent = instance.parent ? instance.parent.type : undefined
+            root.ce._injectChildStyle(type, parent)
           }
 
           if (__DEV__) {

packages/runtime-dom/__tests__/customElement.spec.ts

@@ -916,6 +916,110 @@ describe('defineCustomElement', () => {
       assertStyles(el, [`div { color: blue; }`, `div { color: red; }`])
     })
 
+    test('inject child component styles before parent styles', async () => {
+      const Baz = () => h(Bar)
+      const Bar = defineComponent({
+        styles: [`div { color: green; }`],
+        render() {
+          return 'bar'
+        },
+      })
+      const WrapperBar = defineComponent({
+        styles: [`div { color: blue; }`],
+        render() {
+          return h(Baz)
+        },
+      })
+      const WBaz = () => h(WrapperBar)
+      const Foo = defineCustomElement({
+        styles: [`div { color: red; }`],
+        render() {
+          return [h(Baz), h(WBaz)]
+        },
+      })
+      customElements.define('my-el-with-wrapper-child-styles', Foo)
+      container.innerHTML = `<my-el-with-wrapper-child-styles></my-el-with-wrapper-child-styles>`
+      const el = container.childNodes[0] as VueElement
+
+      // inject order should be child -> parent
+      assertStyles(el, [
+        `div { color: green; }`,
+        `div { color: blue; }`,
+        `div { color: red; }`,
+      ])
+    })
+
+    test('inject child component styles when parent has no styles', async () => {
+      const Baz = () => h(Bar)
+      const Bar = defineComponent({
+        styles: [`div { color: green; }`],
+        render() {
+          return 'bar'
+        },
+      })
+      const WrapperBar = defineComponent({
+        styles: [`div { color: blue; }`],
+        render() {
+          return h(Baz)
+        },
+      })
+      const WBaz = () => h(WrapperBar)
+      // without styles
+      const Foo = defineCustomElement({
+        render() {
+          return [h(Baz), h(WBaz)]
+        },
+      })
+      customElements.define('my-el-with-inject-child-styles', Foo)
+      container.innerHTML = `<my-el-with-inject-child-styles></my-el-with-inject-child-styles>`
+      const el = container.childNodes[0] as VueElement
+
+      assertStyles(el, [`div { color: green; }`, `div { color: blue; }`])
+    })
+
+    test('inject nested child component styles', async () => {
+      const Baz = defineComponent({
+        styles: [`div { color: yellow; }`],
+        render() {
+          return h(Bar)
+        },
+      })
+      const Bar = defineComponent({
+        styles: [`div { color: green; }`],
+        render() {
+          return 'bar'
+        },
+      })
+      const WrapperBar = defineComponent({
+        styles: [`div { color: blue; }`],
+        render() {
+          return h(Baz)
+        },
+      })
+      const WBaz = defineComponent({
+        styles: [`div { color: black; }`],
+        render() {
+          return h(WrapperBar)
+        },
+      })
+      const Foo = defineCustomElement({
+        styles: [`div { color: red; }`],
+        render() {
+          return [h(Baz), h(WBaz)]
+        },
+      })
+      customElements.define('my-el-with-inject-nested-child-styles', Foo)
+      container.innerHTML = `<my-el-with-inject-nested-child-styles></my-el-with-inject-nested-child-styles>`
+      const el = container.childNodes[0] as VueElement
+      assertStyles(el, [
+        `div { color: green; }`,
+        `div { color: yellow; }`,
+        `div { color: blue; }`,
+        `div { color: black; }`,
+        `div { color: red; }`,
+      ])
+    })
+
     test('with nonce', () => {
       const Foo = defineCustomElement(
         {

packages/runtime-dom/src/apiCustomElement.ts

@@ -232,6 +232,8 @@ export class VueElement
   private _styleChildren = new WeakSet()
   private _pendingResolve: Promise<void> | undefined
   private _parent: VueElement | undefined
+  private _styleAnchors: WeakMap<ConcreteComponent, HTMLStyleElement | Text> =
+    new WeakMap()
   /**
    * dev only
    */
@@ -584,6 +586,7 @@ export class VueElement
   private _applyStyles(
     styles: string[] | undefined,
     owner?: ConcreteComponent,
+    parentComp?: ConcreteComponent & CustomElementOptions,
   ) {
     if (!styles) return
     if (owner) {
@@ -592,12 +595,43 @@ export class VueElement
       }
       this._styleChildren.add(owner)
     }
+
+    // if parent has no styles but child does, create an anchor
+    // to inject child styles before it.
+    if (parentComp && !parentComp.styles) {
+      const anchor = document.createTextNode('')
+      const styleAnchor = this._styleAnchors.get(this._def)
+      if (styleAnchor) {
+        this.shadowRoot!.insertBefore(anchor, styleAnchor)
+      } else {
+        this.shadowRoot!.prepend(anchor)
+      }
+      this._styleAnchors.set(this._def, anchor)
+    }
+
     const nonce = this._nonce
+    let last = undefined
     for (let i = styles.length - 1; i >= 0; i--) {
       const s = document.createElement('style')
       if (nonce) s.setAttribute('nonce', nonce)
       s.textContent = styles[i]
-      this.shadowRoot!.prepend(s)
+
+      // inject styles before parent styles
+      if (parentComp) {
+        this.shadowRoot!.insertBefore(
+          s,
+          last ||
+            this._styleAnchors.get(parentComp) ||
+            this._styleAnchors.get(this._def) ||
+            null,
+        )
+      } else {
+        this.shadowRoot!.prepend(s)
+        this._styleAnchors.set(this._def, s)
+      }
+      last = s
+      if (owner && i === 0) this._styleAnchors.set(owner, s)
+
       // record for HMR
       if (__DEV__) {
         if (owner) {
@@ -665,8 +699,11 @@ export class VueElement
   /**
    * @internal
    */
-  _injectChildStyle(comp: ConcreteComponent & CustomElementOptions): void {
-    this._applyStyles(comp.styles, comp)
+  _injectChildStyle(
+    comp: ConcreteComponent & CustomElementOptions,
+    parentComp?: ConcreteComponent,
+  ): void {
+    this._applyStyles(comp.styles, comp, parentComp)
   }
 
   /**