Lwc: Accessing event.relatedTarget throws

[bmblb]

Description

Reading window event's relatedTarget property throws.

Steps to Reproduce

Reproducible on a playground: https://developer.salesforce.com/docs/component-library/tools/playground/f0mKwIvOs/1/edit

Expected Results

event.relatedTarget returning smth, undefined at least

Actual Results

Exception is thrown:

TypeError: Cannot read property 'parentNode' of undefined
    at pathComposer (https://d3nm9grey5nsoo.cloudfront.net/api/runtime/lwc-synthetic-shadow/1.1.13-224.8/dist/synthetic-shadow.js:372:25)
    at retarget (https://d3nm9grey5nsoo.cloudfront.net/api/runtime/lwc-synthetic-shadow/1.1.13-224.8/dist/synthetic-shadow.js:396:23)
    at MouseEvent.get [as relatedTarget] (https://d3nm9grey5nsoo.cloudfront.net/api/runtime/lwc-synthetic-shadow/1.1.13-224.8/dist/synthetic-shadow.js:517:16)
    at compiledPreview.js:694:34
    at fnOrObj.$$lwcEventWrapper$$ (https://d3nm9grey5nsoo.cloudfront.net/api/runtime/lwc-synthetic-shadow/1.1.13-224.8/dist/synthetic-shadow.js:2471:44)

Browsers Affected

Version

• "@lwc/engine: "1.1.13-224.4"

Possible Solution

Additional context/Screenshots
Are there any restrictions to listen window events?

[caridy]

Probably a bug in synthetic shadow dom polyfill. cc @Gr8Gatsby @jodarove @ravijayaramappa

[git2gus[bot]]

This issue has been linked to a new work item: W-7417990

[pmdartus]

It looks like the pathComposer is doing a wrong type guard.

https://github.com/salesforce/lwc/blob/f6844f28894b8bad1336dd1753314e41b1737ac5/packages/%40lwc/synthetic-shadow/src/3rdparty/polymer/path-composer.ts#L39

At this point in the algorithm, the current.parentNode might be an instance of Window. Accessing parentNode on the Window returns undefined explaining why the error is thrown.