The failing test looks like something we should investigate. I think we're duplicating events now, perhaps?

FAILED TESTS:
  details-menu element
    mutually exclusive menu items as labels
      ✖ manages checked state and fires events
        Chrome Headless 92.0.4515.131 (Linux x86_64)
      AssertionError: selected event is fired once: expected 2 to equal 1
          at Context.<anonymous> (test.js:315:14)

Could you add a test for the markup that is buggy?

@srt32 @koddsson The old logic ignored all the click events when inputs were nested in labels, so it wasn't clear that there were 2 of them, and my logic should only care of one and discard the second.

The problem is that a label with a nested input will get the click event twice, once directly and a second one bubbling up from the input (the label always propagates clicks to the input it stands for). stopPropagation wasn't useful preventing this second event, so I added a condition to return if the event is coming from inside a label but not the label itself, as that's always echo.
This was even more necessary because the input mutated in between, that's the reason my logic caused a double event: it is not checked yet in the first event (target: label), but it is already checked as a response to the click in the echoed event (target: input).