Blocks several puppeteer tests relying on browser.pages

  async pages() {
    const pages = await Promise.all(
        this.targets()
            .filter(target => target.type() === 'page')
            .map(target => target.page())
    );
    return pages.filter(page => !!page);
  }

https://github.com/GoogleChrome/puppeteer/blob/v1.6.2/lib/Browser.js#L185-L192

Seems to happen in any puppeteer test, doesn't matter how basic but here's an example of test that will hang on browser.pages:

  console.log("Create page");
  const page = await browser.newPage();

  console.log("Go to example.com");
  await page.goto("http://example.com/");

  console.log("Call browser.pages");
  await browser.pages();

As far as gutenberg is concerned, this probably only impacts preview.test.js

browser.pages() will create Targets for all the available pages, and Page.create contains:

await Promise.all([
      client.send('Target.setAutoAttach', {autoAttach: true, waitForDebuggerOnStart: false}),
      client.send('Page.setLifecycleEventsEnabled', { enabled: true }),
      client.send('Network.enable', {}),
      client.send('Runtime.enable', {}),
      client.send('Security.enable', {}),
      client.send('Performance.enable', {}),
      client.send('Log.enable', {}),
    ]);

https://github.com/GoogleChrome/puppeteer/blob/v1.6.2/lib/Page.js#L48-L56

We hang on client.send('Network.enable', {}). When we try to create NetworkObserver in Network.enable we crash on

    const registrar = Cm.QueryInterface(Ci.nsIComponentRegistrar);
    registrar.registerFactory(
      SINK_CLASS_ID,
      SINK_CLASS_DESCRIPTION,
      SINK_CONTRACT_ID,
      this._channelSinkFactory
    );

https://searchfox.org/mozilla-central/rev/40ef22080910c2e2c27d9e2120642376b1d8b8b2/remote/domains/parent/network/NetworkObserver.jsm#73-79

For what it’s worth, we also see this XPCOM exception trying to
initialise the NetworkObserver when Network.enable is called
in the beforeAll function when running the Puppeteer tests.

I actually fixed that on bug 1585272.