So it seems as though every month or so, some id10t is bit by the Thunderbird bug where it mangles perfectly reasonable looking git-format-patch drafts.
This month, I won the prize! :-)
I had always habitually done --attach, because that's what GitWine says to do. But after scratching my head, reading man pages, and checking my drafts, I decided that was lame; inline is always nicer, and I dropped the --attach. Big mistake; the draft may look okay, but the end result is mangled somehow.
Thanks to Dmitry for alerting me.
I've filed a bug with Mozilla over this: https://bugzilla.mozilla.org/show_bug.cgi?id=435020
My sense is that its a subtle bug of some kind; I've had a hard time articulating a strategy to reproduce it (which, by Occam's razor, immediately suggests I'm just missing something :-/).
So if others are bit by this and can share any clues or tips for the Mozilla devs on how to reproduce it, I'd appreciate it.
Cheers,
Jeremy