#397 no Display Update on location change

[Vince Calhoun]

Make a large block....(e.g. use New-Block and create a
128x128x300 block using the function (lambda (x)
(random-uniform))

Hit page-up....you will notice that sometimes the
screen doesn't update. If that occurs and the window
is resized, the screen then displays the new slice.

VDC