fix(runtime-core): Avoid mutating original options obj in createApp

packages/runtime-core/src/apiCreateApp.ts

@@ -179,6 +179,9 @@ export function createAppAPI<HostElement>(
   hydrate?: RootHydrateFunction
 ): CreateAppFunction<HostElement> {
   return function createApp(rootComponent, rootProps = null) {
+
+    rootComponent = { ...rootComponent }
+
     if (rootProps != null && !isObject(rootProps)) {
       __DEV__ && warn(`root props passed to app.mount() must be an object.`)
       rootProps = null

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

@@ -12,4 +12,27 @@ 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 options object', () => {
+
+    const originalObj =  {
+      data() {
+        return {
+          counter: 0
+        }
+      }
+    }
+
+    const handler = jest.fn((msg, instance, trace) => {
+      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')) 
+    expect(originalObj).toMatchObject(Root) // ensure no additional properties are added to Root
+  })
 })