fix svg foreignObject regression (fix #4478)

Author: yyx990803

src/core/vdom/create-element.js

@@ -71,7 +71,6 @@ export function _createElement (
       // unknown or unlisted namespaced elements
       // check at runtime because it may get assigned a namespace when its
       // parent normalizes children
-      ns = tag === 'foreignObject' ? 'xhtml' : ns
       vnode = new VNode(
         tag, data, children,
         undefined, undefined, context
@@ -91,6 +90,10 @@ export function _createElement (
 
 function applyNS (vnode, ns) {
   vnode.ns = ns
+  if (vnode.tag === 'foreignObject') {
+    // use default namespace inside foreignObject
+    return
+  }
   if (vnode.children) {
     for (let i = 0, l = vnode.children.length; i < l; i++) {
       const child = vnode.children[i]

src/platforms/web/util/element.js

@@ -5,8 +5,7 @@ import { makeMap } from 'shared/util'
 
 export const namespaceMap = {
   svg: 'http://www.w3.org/2000/svg',
-  math: 'http://www.w3.org/1998/Math/MathML',
-  xhtml: 'http://www.w3.org/1999/xhtml'
+  math: 'http://www.w3.org/1998/Math/MathML'
 }
 
 export const isHTMLTag = makeMap(

test/unit/modules/vdom/create-element.spec.js

@@ -132,6 +132,15 @@ describe('create-element', () => {
     expect(vnode.children[0].componentOptions).toBeUndefined()
   })
 
+  it('render svg foreignObject with correct namespace', () => {
+    const vm = new Vue({})
+    const h = vm.$createElement
+    const vnode = h('svg', [h('foreignObject', [h('p')])])
+    expect(vnode.ns).toBe('svg')
+    expect(vnode.children[0].ns).toBe('svg')
+    expect(vnode.children[0].children[0].ns).toBeUndefined()
+  })
+
   it('warn observed data objects', () => {
     new Vue({
       data: {