I agree, this is very irritating, and has been complained about quite a bit here. It only causes a problem when the line being completed is at the bottom of the editor pane, so one work-round is not to let the line get to the bottom! You can make the editor do this by using the less than clearly named Settings|Editor|Editor Tweaks|Editor Caret Buffer option, which keeps the editor insertion point away from the bottom of the editor.