ChangeScreen2: Allow scrolling while reply box is open

Permit the user to scroll the page to look at the history while typing
a reply.  This can make it easier to reference text written by another.

GWT supports setting the popup non-modal but using glass with an empty
style to get modal like behavior without breaking basic page features
like browser managed scrolling.

Change-Id: I7d29387baa4af88650a9227f61bf549fc223466e