LazyBlock docs and physics
LazyBlock docs and physics

This patch adds dartdoc to LazyBlock. Also, this patch fixes the scrolling
physics of LazyBlock. Previously, we updated a running simulation only when the
change in scroll behavior changed the current scroll offset. Now we update
running simulations every time the behavior changes because the simulation
might depend on quantities other than the current scroll offset.
spritewidget/spritewidget,spritewidget/spritewidget,spritewidget/spritewidget