Are you using 14? There's a setting in 14 that's different for scrollbars but I thought it was just on fields and not portals.
Whenever I have a rendering problem in WebDirect, I usually use the inspector to remove all styles from the object (in your case the portal), and seeing if it will render fine without styles. Then I add styles back as needed until I either break (and then don't use) or correct a style.
Some screenshots of your solution might help with a diagnosis as well.
my thought is that it might be going off because you're using a portal inside a tab object. I try to avoid tabs at all costs in WD just because it's not really something I'd normally put on a website.
Try dragging your portal outside the tab object to see if that corrects it.
Have you also checked other browsers like chrome and firefox to make sure it's not browser specific?
that's it !
Thanks and best regards - Dick