Previously, when a user view the message source of a message at the
very top with the blue box around, the tooltip for the button will
be partially hidden by the recipient bar. Ths cause is some legacy
CSS from, for example, 3cd33c0fea,
which increased the z-index for the bodies of selected messages.
The intent of that code appeared to be something around handling overlaps
between unread indicators and the blue selected message box. It's logically
incorrect, and testing demonstrates that the blue box works fine next to unread
messages without this change, so we can safely remove these z-index values.
c8d043e12c