Bug 1080820 - [Messages][RTL] Sticky header does not take the whole width

- Adjust sticky header position and force background color in white instead of transparent.
Some thoughts about this:

* I think we want to have "visibility: hidden" instead of "display: none" so that we don't have a reflow but only a repaint. But maybe this is not necessary to hide it (keep reading)
* we can toggle a class here, instead of setting "hidden". What about `sticky.classList.toggle('stick-has-content', display)` ?
* in each app this is used, this class can be used to set the background-color to white if necessary -- or to switch visibility from hidden (in the #sticky CSS) to visible (in the #sticky.stick-has-content CSS).

What do you think?