Instead of scrolling (which can be done, BTW), why not "filter" the list so that the list displays only the records that match the text you have entered? That's easier to implement.
You can set up a text field just above the portal, I'll name it Search. Then you can give the portal this filter expression:
YourTable::Search = Left ( YourTable::SetName ; Length ( YourTable::Search ) )
Write this simple script:
Refresh Window [Flush cached join results]
Set Selection [YourTable::Search ; start position: Length ( YourTable::Search ) + 1 ; end position: 0 ]
And use the OnObjectModify Script trigger on the search field to perform this script which forces the filtered portal to update with each keystroke.
Here's demo file you can download with two different versions of this selection portal plus two enhanced implementations of auto-complete enabled dropdown lists that enter ID numbers instead of names: http://www.4shared.com/file/umvOy55n/EnhancedValueSelection.html