Comment 4 for bug 627106

The problem is in gwui.MessageStream.on_viewport_scroll(). handle_scroll() is called with a value which may not be within the range of the scrollbar, and therefore the range of messages, which causes it to revert to showing the top of the page. It also causes the gap when you scroll up when at the top message. The scrollbar automatically corrects for this when it is set, but the out-of-bounds value is sent to handle_scroll(). Attached a patch which sends scrollbar.get_value() instead.