Simple proof of concept: http://jsfiddle.net/rVxkv/

This opens 2 Thunderbird windows in Firefox 26 but only one in Chromium 31.0.1650.63.

edit: I totally agree it shouldn't be possible :)

I've never seen a prompt to open a mailto: link. Where is it specified there should be one?

I think by "interactive prompt" he means "user interaction".

I suppose I can see an argument for popping something up before firing the schema handler when something sets location.href to a mailto: URL, but that seems like the sort of thing where you'd really want to wait for evidence that it's a problem for anyone before you implement it; it both annoys the user and complicates your code, neither of which is desirable in the absence of real provocation.