The definition for onClose as part of EuiFlyout is () => void. I was taking advantage of this by having my not-close button invoke the same method with an optional parameter.

However, I was surprised when certain closure's were triggering issues in my code. Upon inspection, it kind of became clear that I was receiving the events from the button clicks:

This is easy to workaround: just pass in the function as () => actualCallback(), but it was definitely unexpected.