I saw now the "Design Guidelines, 2.1.1 Navigating within the system user interface
" actually does specify that Page Backward/Forward is the expected behavior for <</>>, hence current roListScreen mis(ing)behavior may be (up)graded as a bug
Till fancy future when RokuCo fixes this, here is a "band-aid" per TheEndless's idea:
rowNo = msg.getIndex() 'track where cursor is at
btn = msg.GetIndex()
if btn = 8: '<<
ls.setFocusedListItem( max(rowNo-5, 0) )
elseif btn = 9: '>>
ls.setFocusedListItem( min(rowNo+5, content.count()-1) )
The patch has issues of its own: it cannot auto-repeat of << and >>, so cannot step on a button to go to the beginning or end of the list. And pressing these buttons makes the cranky "error" sound instead of the milder arrow up/down sound.
I said +/-6 earlier but the right advance value is +/-5, considering that only 5 whole lines are seen mid-list, plus half top and half bottom item.