Tab completion on long line puts lineview just before completed word
Sorry for the very weird and not very useful title, but that is the only way I describe the problem in one sentence :).
When you type a long line when you want to say something, and that line doesn't fit in the window, then the line scrolls. When you want to tab complete a nickname there the tab completion works perfectly, only the view of the typed text changes to show the text just before the completed word. So you don't see the completed word anymore.