Note the region outlined in black in the lower left. That appears in response to a mouseenter event when I hit “Download PNG” from the three dot “kebab” menu. Thus, I suppose it shouldn’t be there.
You can take a look at the notebook to get a better idea of what I’m saying:
Ah wow that sounds annoying! I just tried a few times in Chrome/Safari/Firefox; it worked fine most of the time, but I did get the effect you’re seeing once. I think if you make sure you’ve mouseleft before opening the menu, and then don’t move for a second after you click “Download PNG”, it should work. I don’t think the mousemoves are propagating through to behind the menu while your moue is on it, at least not for me, but when I play with it it’s easy for a stray move to leave the effect on while the PNG is generated. I could be wrong.