@lwc/synthetic-shadow installed even when native Shadow DOM is available

[aputinski]

Looks like this check is returning undefined even when native Shadow DOM is available (JSDOM 16).

https://github.com/salesforce/lwc-test/blob/master/packages/%40lwc/jest-preset/src/setup.js#L9

I'm guessing $constructorCache$ was changed at some point in JSDOM.

Our LWC components are used with native Shadow DOM, so we have a few tests that check things like assignedNodes()

[pmdartus]

Shadow DOM has been introduced in jsdom 2 years ago with the 12.2 release. JSDOM 16.0 adds support for custom elements. So far synthetic shadow was always enabled regardless if native shadow DOM was supported on not by jsdom. Based on this, I don't think the $constructorCache$ is linked to your issue.

@aputinski Could you please share a minimal repro of your issue for us to investigate further?

Note: As of jsdom 16, the DOM constructors and prototypes are recreated from scratch. This $constructorCache$ is not needed anymore.

[aputinski]

So far synthetic shadow was always enabled regardless if native shadow DOM was supported on not by jsdom.

@lwc/synthetic-shadow seems to be conditionally required based on that check?

Here's the test case:
https://github.com/aputinski/lwc-jest-preset-test-case

[pmdartus]

I gave a look into this specific issue and I understand better the source of this issue. @lwc/jest-preset always enables the synthetic shadow today to match what is currently experience in Lightning. Currently, LWC component using the synthetic shadow doesn't support slotting for the top-level component. This is why assignedNodes() doesn't return the right result in the test.

IMO, we should add a config flag to turn off synthetic shadow in the case where it is not needed.