We add a box shadow at top of the recipient bar to hide message
text that are partially visible above the recipient bar. At 100%
zoom, this issue is not visible but it has been reported by a user
at 150% zoom.
Following this change we don't need to do change any of our JS
calculations since `y` position of header remains the same
and when determining if a header `is_sticky` that is the only
thing we verify.
7dacbc934f