avoid 'app not found' flickering while awaiting for mount (#56483)

src/core/public/application/ui/app_container.test.tsx

@@ -0,0 +1,88 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import React from 'react';
+import { mount } from 'enzyme';
+
+import { AppContainer } from './app_container';
+import { Mounter, AppMountParameters, AppStatus } from '../types';
+
+describe('AppContainer', () => {
+  const appId = 'someApp';
+  const setAppLeaveHandler = jest.fn();
+
+  const flushPromises = async () => {
+    await new Promise(async resolve => {
+      setImmediate(() => resolve());
+    });
+  };
+
+  const createResolver = (): [Promise<void>, () => void] => {
+    let resolve: () => void | undefined;
+    const promise = new Promise<void>(r => {
+      resolve = r;
+    });
+    return [promise, resolve!];
+  };
+
+  const createMounter = (promise: Promise<void>): Mounter => ({
+    appBasePath: '/base-path',
+    appRoute: '/some-route',
+    unmountBeforeMounting: false,
+    mount: async ({ element }: AppMountParameters) => {
+      await promise;
+      const container = document.createElement('div');
+      container.innerHTML = 'some-content';
+      element.appendChild(container);
+      return () => container.remove();
+    },
+  });
+
+  it('should hide the "not found" page before mounting the route', async () => {
+    const [waitPromise, resolvePromise] = createResolver();
+    const mounter = createMounter(waitPromise);
+
+    const wrapper = mount(
+      <AppContainer
+        appId={appId}
+        appStatus={AppStatus.inaccessible}
+        mounter={mounter}
+        setAppLeaveHandler={setAppLeaveHandler}
+      />
+    );
+
+    expect(wrapper.text()).toContain('Application Not Found');
+
+    wrapper.setProps({
+      appId,
+      setAppLeaveHandler,
+      mounter,
+      appStatus: AppStatus.accessible,
+    });
+    wrapper.update();
+
+    expect(wrapper.text()).toEqual('');
+
+    resolvePromise();
+    await flushPromises();
+    wrapper.update();
+
+    expect(wrapper.text()).toContain('some-content');
+  });
+});

src/core/public/application/ui/app_container.tsx

@@ -53,25 +53,27 @@ export const AppContainer: FunctionComponent<Props> = ({
         unmountRef.current = null;
       }
     };
-    const mount = async () => {
-      if (!mounter || appStatus !== AppStatus.accessible) {
-        return setAppNotFound(true);
-      }
 
-      if (mounter.unmountBeforeMounting) {
-        unmount();
-      }
+    if (!mounter || appStatus !== AppStatus.accessible) {
+      return setAppNotFound(true);
+    }
+    setAppNotFound(false);
 
+    if (mounter.unmountBeforeMounting) {
+      unmount();
+    }
+
+    const mount = async () => {
       unmountRef.current =
         (await mounter.mount({
           appBasePath: mounter.appBasePath,
           element: elementRef.current!,
           onAppLeave: handler => setAppLeaveHandler(appId, handler),
         })) || null;
-      setAppNotFound(false);
     };
 
     mount();
+
     return unmount;
   }, [appId, appStatus, mounter, setAppLeaveHandler]);