fix(compat): fix app-level asset registration affecting other local apps (#5979)

Author: snoozbuster

packages/runtime-core/src/compat/global.ts

@@ -381,9 +381,10 @@ function installLegacyAPIs(app: App) {
 
 function applySingletonAppMutations(app: App) {
   // copy over asset registries and deopt flag
-  ;['mixins', 'components', 'directives', 'filters', 'deopt'].forEach(key => {
+  app._context.mixins = [...singletonApp._context.mixins]
+  ;['components', 'directives', 'filters'].forEach(key => {
     // @ts-ignore
-    app._context[key] = singletonApp._context[key]
+    app._context[key] = Object.create(singletonApp._context[key])
   })
 
   // copy over global config mutations

packages/vue-compat/__tests__/global.spec.ts

@@ -448,3 +448,25 @@ test('global asset registration should affect apps created via createApp', () =>
   expect(vm.$el.textContent).toBe('foo')
   delete singletonApp._context.components.foo
 })
+
+test('post-facto global asset registration should affect apps created via createApp', () => {
+  const app = createApp({
+    template: '<foo/>'
+  })
+  Vue.component('foo', { template: 'foo' })
+  const vm = app.mount(document.createElement('div')) as any;
+  expect(vm.$el.textContent).toBe('foo')
+  delete singletonApp._context.components.foo
+})
+
+test('local asset registration should not affect other local apps', () => {
+  const app1 = createApp({});
+  const app2 = createApp({});
+
+  app1.component('foo', {});
+  app2.component('foo', {});
+
+  expect(
+    `Component "foo" has already been registered in target app`
+  ).not.toHaveBeenWarned()
+})