Bug 912028 - Background is visible on keyboard pop in

This fixes [#912028](https://bugzilla.mozilla.org/show_bug.cgi?id=912028). The current code tried to address this issue already but made two wrong assumptions:

1. If `hide` was not in classList of keyboard the height would be updated directly. It didn't take into account that the open transition might be happening at that time and the height would be incorrect.
2. Listens to first transform event that comes in, and that is the visibility transition; not the transform (which we should be waiting for).
3. (CSS) We ease on visibility but that doesn't have an ease. Should do it on opacity.

So those are the issues addressed. I've written tests around it because it felt quite fragile when touching it; and it helps with quickly testing my assumptions as well :-)