I have a portal where tabbing from the last field on a row takes me out of the portal to a field elsewhere, rather than going to the next portal row. What is causing this behavior? (I feel like I'm missing something obvious.)

Actually, there is only only one field on the portal rows that allows entry and is in the tab order. The idea is to tab from portal row to portal row, staying in this field. If I allow entry into another field on the portal row and put it in the tab order, this does not change the behavior.

The field is in the layout's tab order; not the last object in the tab order. The portal is based on relationship that allows creation of new records. The portal is not sorted or filtered. No script triggers; though, when I apply a trigger -- onObjectExit perform script Go To Portal Row [Next] -- that makes no difference. Tabbing out of the field runs the script, then the focus goes to a field outside the portal.  

Tabbing down the portal rows does not work if you are in a list view. This behavior only occurs in a detail view (or with onObjectKeystroke script triggers)