I think hasCard and tellCard seem like the right way to go.  Anything we can do to reduce/eliminate reflows is going to be very helpful to us, regardless of flicker.  (But flicker is indeed bad.  I think we can avoid it even with cramming a folder card to the left of the empty message card, see my review comments on Juliens' patch at: https://bugzilla.mozilla.org/show_bug.cgi?id=843768#c18 .  Of course, you probably better understand the card DOM stuff better than I now. :)