A.L.E.C wrote:
Original message contains "div {display:none;}" style, which is replaced by Roundcube with "#messagebody div {display:none;}". This is a bug, because #messagebody div already contains another div (class="message-htmlpart"). That's why the whole message contents are not displayed. I'll work on this.
Actually there was also <div class="rcmBody">. Fixed.
http://trac.roundcube.net/changeset/3410