Bug 1023658 - [User story] Hide CC and BCC fields while composing an ema...

...il
Setting this to 0.6s makes it feel a bit better. At 0.2s, it is too choppy when I try on the flame device. I think the big thing is that the overall height gets adjusted first, then have the animation slide in. At 0.2, the height is still getting adjusted, and the effort to push the rest of the card down eats up the animation performance.