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.
Comments
It shouldn't be possible to launch mailto links without an interactive prompt; if it is, please file a bug on the browser.
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.