Due to the amount of data that _may_ be displayed (not that it
necessarily makes sense), I believe using an NSOutlineView is
probably ruled out due to it's reliance on one object per row (I
don't want to have to allocate 100,000+ objects).
NSOutlineView only needs objects for visible rows. The dataSource API
allows the implementor to be extremely lazy about instantiating row
data. There shouldn't be any problem with huge collections, as long as
your data source is careful about how it works.
(I think the minimum number of row objects necessary to be in memory
is one per currently-open "folder", plus one more for the row being
drawn at that instant.)
