A maximised composer goes position: fixed but stays a child of the dock,
and had no z-index of its own. The dock is a stacking context, so the
positioned parts of any composer later in the DOM (its recipients row, its
editor) painted straight over the full-screen one.
Give the maximised composer its own layer, and hide the other composers
while one is full screen: they cannot be reached anyway, and the 24px
inset would otherwise show their footers along the bottom edge. They stay
mounted, so nothing being written in them is lost.
Fixes#330
jcoffey-dev