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Thanks for the PR!

The most important part here is how to test this change, though, the source change itself is simple and it has already been suggested by @gibson042 in #4417.

Do you want to write a test?

jQuery only runs unit tests in CI, we don't have any integration test setup. This poses problems in situations like this one as some focus-related bugs are specific to native events and emulating the behavior in unit tests is non-trivial.

I don't know how to write the test, I tried for a while in the past but I didn't manage to do it before I run out of time. So I can't provide a very concrete advise, unfortunately.

You would need to add a test in test/unit/event.js, you can have a look at existing focus tests there, maybe you'll find some inspiration.

@radar @mgol After several months It still broken even if the fix is known. I hope it will be merged soon.
We currently are not able to update jQuery because lot of our tests fail on this valueof undefined.

@ryuran We still need to figure out a test for this. If you can help with that then the PR will surely be merged sooner.

I submitted a version with a test so I'll close this PR. Sorry! See #4799.