if i cat a long file (eg: /etc/services), and scroll back through it with scrollbar or shift-pgup, large empty spaces are periodically inserted into the display where there were none. other terminals (xterm, rxvt, gnome-terminal) don't display this empty space.
Try setting REFRESH_PERIOD in src/feature.h to 1 or 2 before compile if this really bothers you. It will slow things down, but it will prevent those large blank spaces. Personally, I prefer the speed. :-) HTH, Michael