Shoutbox

nah, Plus has nothing to do with that, it's messenger who causes the problem... Anyway, this is what you have to do when that happens:
Scroll down the conversation.
Click, with the mouse, at the end of the text in the conversation, the same as if you wanted to put the cursor there to start writting
Click now in the zone where you write messages.
That's all. From then, the conversation should scroll as usual.