The screen sharing window is within the browser window. The default size is the same as the whiteboard size.
Because you can not resize the screen sharing window, it covers the chat window and so you have difficultly chatting and screen sharing.
The workaround is to start the session with a small browser window and then expand the browser window so that you can move the screen share around.
The version is the one running on http://demo.openmeetings.de/openmeetings/ at 2012 07 05