adds explanation comment and test

packages/server/lib/browsers/chrome.ts

@@ -146,6 +146,12 @@ let browserCriClient: BrowserCriClient | undefined
  * @param userDir
  */
 const _getChromePreferences = (userDir: string): Bluebird<ChromePreferences> => {
+  /**
+   * By returning empty objects here, the user is indicating that they do not want
+   * Chrome preferences read. If they do not provide preferences via config,
+   * this effectively prevents Cypress from writing to the Chrome preferences files.
+   * See: _writeChromePreferences
+   */
   if (process.env.IGNORE_CHROME_PREFERENCES) {
     debug('ignoring chrome preferences...')
 
@@ -598,13 +604,16 @@ export = {
       launchOptions.preferences = _mergeChromePreferences(preferences, launchOptions.preferences as ChromePreferences)
     }
 
+    const p = _disableRestorePagesPrompt(userDir)
+
+    debug(p)
+    await p
     const [extDest] = await Bluebird.all([
       this._writeExtension(
         browser,
         options,
       ),
       _removeRootExtension(),
-      _disableRestorePagesPrompt(userDir),
       // Chrome adds a lock file to the user data dir. If we are restarting the run and browser, we need to remove it.
       fs.unlink(path.join(userDir, 'SingletonLock')).catch(() => {}),
       _writeChromePreferences(userDir, preferences, launchOptions.preferences as ChromePreferences),

packages/server/test/unit/browsers/chrome_spec.js

@@ -192,6 +192,37 @@ describe('lib/browsers/chrome', () => {
       })
     })
 
+    context('when IGNORE_CHROME_PREFERENCES env is set', () => {
+      let oldPref
+      let writeJson
+
+      beforeEach(function () {
+        oldPref = process.env.IGNORE_CHROME_PREFERENCES
+        process.env.IGNORE_CHROME_PREFERENCES = true
+        this.readJson.rejects({ code: 'ENOENT' })
+        writeJson = sinon.stub(fs, 'outputJson').resolves()
+      })
+
+      afterEach(() => {
+        process.env.IGNORE_CHROME_PREFERENCES = oldPref
+        writeJson.restore()
+      })
+
+      it('does not write preferences', async function () {
+        chrome._writeExtension.restore()
+        utils.getProfileDir.restore()
+
+        await chrome.open({
+          isHeadless: true,
+          isHeaded: false,
+          name: 'chromium',
+          channel: 'stable',
+        }, 'http://', openOpts, this.automation)
+
+        expect(writeJson).not.to.be.called
+      })
+    })
+
     it('DEPRECATED: normalizes --load-extension if provided in plugin', function () {
       plugins.registerEvent('before:browser:launch', (browser, config) => {
         return Promise.resolve(['--foo=bar', '--load-extension=/foo/bar/baz.js'])