Select the portal and then from file menu, select Arrange > Send to Back.
FileMaker, even if the fields are lower in stacking order (behind the portal) will display the fields in browse mode. Also make sure that the fields exist only in the top row and that they are totally within the row. Particularly stay away from the top line by 1 px and the left side by 1 px.
An easy way to "tweak" an objects postion by just one or two pixels is to select it in layout mode and then press an arrow key to "nudge" it around one pixel at a time.