fix: Prevent mocha tests failures when window does not have focus. (#9332)

* chore: Add puppeteer-core as a dev dependency.

* fix: Make mocha tests run in a fake-focused window.

* fix: Make `test:mocha:interactive` use the same gulp codepath as `test`.

tests/mocha/webdriver.js

@@ -15,9 +15,12 @@ const {posixPath} = require('../../scripts/helpers');
  * Runs the Mocha tests in this directory in Chrome. It uses webdriverio to
  * launch Chrome and load index.html. Outputs a summary of the test results
  * to the console.
+ *
+ * @param {boolean} exitOnCompletetion True if the browser should automatically
+ *     quit after tests have finished running.
  * @return {number} 0 on success, 1 on failure.
  */
-async function runMochaTestsInBrowser() {
+async function runMochaTestsInBrowser(exitOnCompletion = true) {
   const options = {
     capabilities: {
       browserName: 'chrome',
@@ -45,6 +48,17 @@ async function runMochaTestsInBrowser() {
   console.log('Loading URL: ' + url);
   await browser.url(url);
 
+  // Toggle the devtools setting to emulate focus, so that the window will
+  // always act as if it has focus regardless of the state of the window manager
+  // or operating system. This improves the reliability of FocusManager-related
+  // tests.
+  const puppeteer = await browser.getPuppeteer();
+  await browser.call(async () => {
+    const page = (await puppeteer.pages())[0];
+    const session = await page.createCDPSession();
+    await session.send('Emulation.setFocusEmulationEnabled', { enabled: true });
+  });
+
   await browser.waitUntil(async() => {
     const elem = await browser.$('#failureCount');
     const text = await elem.getAttribute('tests_failed');
@@ -74,7 +88,7 @@ async function runMochaTestsInBrowser() {
   if (parseInt(numOfFailure) !== 0) {
     return 1;
   }
-  await browser.deleteSession();
+  if (exitOnCompletion) await browser.deleteSession();
   return 0;
 }