fix(runtime-core): Avoid mutating original options object in createApp (#4840)

fix #4398

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

@@ -12,4 +12,33 @@ describe('createApp for dom', () => {
     expect(root.children.length).toBe(1)
     expect(root.children[0] instanceof SVGElement).toBe(true)
   })
+
+  // #4398
+  test('should not mutate original root component options object', () => {
+    
+    const originalObj =  {
+      data() {
+        return {
+          counter: 0
+        }
+      }
+    }
+
+    const handler = jest.fn(msg => {
+      expect(msg).toMatch(`Component is missing template or render function`)
+    })
+
+    const Root = { ...originalObj}
+    
+    const app = createApp(Root)
+    app.config.warnHandler = handler
+    app.mount(document.createElement('div')) 
+ 
+    // ensure mount is based on a copy of Root object rather than Root object itself 
+    expect(app._component).not.toBe(Root)
+    
+    // ensure no mutation happened to Root object
+    expect(originalObj).toMatchObject(Root)
+    
+  })
 })