This is a very subtle bug that is probably very unlikely to be hit in real life but was being hit in the tests (https://github.com/elastic/kibana/issues/15418) because the clicking is so much faster than a person.
You can see in this paused screenshot that the click events haven't finished being added to the path slices, but the pie chart is still visible on the screen:

Not a high priority item, but in a perfect world, it probably shouldn't be allowed to happen at all. Not sure there is an easy fix for it either, but just want to let @elastic/kibana-visualizations team aware of it. I'll find a work around around for the tests.
thanks @Stacey-Gammon
So the problem is that the render-complete has fired, but that there is still initialization going on (ie. clickhandlers being attached?).
I don't think render complete has fired yet, but to the user it shouldn't matter, right? If the user sees a pie chart on the screen, those slices should be clickable, right? Not like, oh you have to wait a few ms before those become clickable? It really isn't an issue if it's extremely fast, but you'd think the click handlers would be attached in the same cycle that the pie chart is displayed so they are already applied.
That is the case with all the charts. we were discussing this with @timroes a while back. Chart should probably be grayed out and not clickable until render complete fires.
We are not able to reproduce this at all. Also appending the SVG elements and attaching the DOM listeners is done synchronous (in the same execution slot). There should be no way any event get in between the SVG showing up and the handlers being registered. Maybe the async failure is in some other part of Kibana (passing on the click event or filter handling).
After discussions with @Stacey-Gammon we will close this issue, since she already made a work around using the render-counter during tests and it's not reproducible locally at all.
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I don't think render complete has fired yet, but to the user it shouldn't matter, right? If the user sees a pie chart on the screen, those slices should be clickable, right? Not like, oh you have to wait a few ms before those become clickable? It really isn't an issue if it's extremely fast, but you'd think the click handlers would be attached in the same cycle that the pie chart is displayed so they are already applied.