Support `chrome --headless`

[brian-mann]

Change 1.

With Chrome 63 coming out we can make some big changes at Cypress.

1. We will be able to support the debugger protocol which will enable native events
2. We will be able to support chrome --headless

The reason we currently do not support chrome --headless is because Chrome does not support extensions in that mode. We use an extension to automate the browser and that's why its a no-go. By switching from the extension to the debugger protocol, we'll be able to get around this.

Change 2.

Once we support chrome headless I propose the following changes to the CLI:

cypress run

Currently cypress run will launch Electron headlessly. This is confusing for users because they can't see what's going on and because they don't read our docs they don't understand that we're launching Electron instead of Chrome. Due to the browser version differences, they see failures in Electron that aren't happening in Chrome.

What I propose is to change this behavior so that cypress run by default will search for Chrome and launch it in headed mode. When no Chrome is installed, it'll fall back to Electron.

Currently we support a --headed option that only applies to Electron. This was done to prevent breaking changes.

This should now switch to being the default but then enabling the user to explicitly opt into the --headless mode for both Electron + Chrome.

This will be a breaking change and could come out in Cypress 2.x.x.

[tnhu]

This is important. Today I found a couple of our tests failing in CI environments but not locally using cypress app. It turns out Chrome and Electron act differently.

Is there anyway you can prioritize this feature? 2.x.x seems like too far away.

[brian-mann]

Chrome and Electron act the same - the difference is the underlying version of Chromium each is using.

Electron uses Chromium 53. Your local version of Chrome likely differs. These different versions will not act exactly the same.

I can't say when this will be prioritized because all of our team members are currently working on other features. We are open source now and are accepting PR's of course. It's going to be a lot of work - likely weeks for someone to work on full time.

As for the 2.x.x - its not that far off because we use semver. Whenever we have the next breaking change we will reach 2.x.x - it could be the next release for that matter!

[brian-mann]

What I would suggest is install chrome in CI and make it match the version you're running locally. We have several docker containers with this prebuilt.

Alternatively you could also run Electron locally to suss out the differences in failing tests. Plenty of users do either.

[nrutman]

@brian-mann, has there been any consideration to either:

1. Expose arguments to pass to chrome to run it in headless mode?
2. Semi-regularly update the version of Chromium used in the Electron browser? It looks like it's currently 10 versions behind.

Those might be lower-hanging (?) fruit that might help close the gap.

[brian-mann]

@nrutman there is already an API for doing your first bullet item: https://docs.cypress.io/api/plugins/browser-launch-api.html

The problem is that running in headless mode disables chrome extensions which prevents Cypress from working. However you can modify other arguments.

Yes, per your second bullet item we have been waiting for Electron to drop 1.8 which bumps node to 8.x.x. They are currently in beta with semi regular releases.

https://electronjs.org/releases#1.8.2-beta.4

[Graham42]

Looks like should be able to load extensions for headless chrome with flags

--disable-extensions-except=/path/to/extension/
--load-extension=/path/to/extension/

Source: puppeteer/puppeteer#659 (comment)

[brian-mann]

I had read somewhere about chrome headless not ever going to support extensions. I don't believe there are any configuration options that will change this. Puppeteer here is irrelevant since this is a restriction from Chrome itself.

[rsudarson]

@brian-mann When this change will be rolledout