jest-environment-jsdom: stop setting document to null on teardown

[jsnajdr]

Removes redundant bit of code that sets global.document to null when jest-environment-jsdom is tearing down.

This change is unblocking JSDOM upgrade in #13825. Actually #13825 already makes the same change, my PR is only adding some unit tests and reasoning around this.

The "set document to null" code was originally added in #5955, to prevent a document load event handler to be fired after the document shuts down. But it turns out, and my testing confirms that, that window.close() already solves this, because in JSDOM window.close() will remove all event listeners from both window and document.

JSDOM didn't always do that correctly, there is a bugfix PR from 2016 that fixed that: jsdom/jsdom#1479 But Jest's #5955 is from 2018, two years later, so I'm wondering which version of JSDOM was it using at the time.

JSDOMEnvironment's teardown method already calls this.global.close(), and that's all that needs to be done. I added a new unit test to verify that the load handler is indeed not being called after env.teardown()/window.close().

Another related PR was #11871, fixing a bug where document was being set to null before calling window.close(). window.close() removes all elements from the document, and when there are custom elements, their disconnectedCallback methods are called. Synchronously. At the time when disconnectedCallback was called, document was already null which was bad and broke things. So #11871 changed the order of the statements: set document to null only after window.close.

I added a unit test also for this. Replacing the original test, which was testing that document is not yet null when window.close() is called, i.e., was testing implementation internals, my new test is testing the real thing: whether a custom element's disconnectedCallback is really called during env.teardown(), and whether it's seeing a valid window and document.

[SimenB]

Yeah, makes sense to me. Could you add a changelog entry?

[jsnajdr]

Could you add a changelog entry?

✅

[SimenB]

https://github.com/facebook/jest/releases/tag/v29.5.0

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