listbase: Fix a problem with rubberbands

listbase: Fix a problem with rubberbands

When I changed things to only collect the set
in the stop() function, I overlooked that this
has the side-effect of only handling items which
are backed by a widget at the time stop() is called.
If we make a big rubberband and autoscroll down too
far, we loose the items that go out the visible range
at the top. Fix that by maintaining the set as we go.