This intends to optimize the case where the top of the scrolling region
is the top of the screen. In theory, scrolling in this case can be
optimized to shifting the start/end of the visible region, and then
rearranging any lines that were not supposed to be scrolled (at the
bottom of the region). However, this didn't produce quite the speedup I
expected.
In addition to a marginal performance improvement, this simplifies some
logic in the Term implementation since now the Grid fully handles row
recycling.
Joe Wilm