Bug 828266 - [email] black flash / email flickers when transitioning from list to email and back - part 2

What I believe to be a slight improvement over the current fix for this bug:

https://github.com/mozilla-b2g/gaia/commit/5c1050aeab1cc04ed49f4759b308b35f1852eadd

This fix does not require an extra intermediate "start" style applied with a reflow kick. Instead, it uses calc() to place the non visible cards onscreen by half a pixel, and that is the style they keep when inserted/ended animation. 

The cards do not show visibly on the screen, and it avoids the black flashing. I believe this approach helps with the loss of animation when going back to the message list. But I have probably been staring at this for too long.

With this patch, I only see a loss of animation frames if the platform-provided vertical scrollbar is on screen or on its way to disappearing when the animation starts. 

So to improve the animation for those cases, I believe it will need some platform help/guidance.