Please see the post I just put up on another thread about portal behavior. I'm sure the two behaviors are related.
A simpler work around would be for your button's script to use go to portal row[select; no dialog; get (portalRowNumber)] to select the row.
^^ Of course. Thanks, Phil. I did not realize that the select would highlight the portal row, but it does.
I did find that I had to put a commit step at the end of the script or I ended up with a very weird bit of UI detritus on the portal (looked like the cursor was in a field that wasn't actually there! --> very odd!! --> it went away as soon as I clicked on the portal...). That worked a charm to clean up the UI.