What version of FileMaker are you using?
What do you mean by the "portal is selected"? (how does the appearance change?)
What do you want to happen when you click in the portal?
Using Filemaker 12 Advanced. 12.0 v3. I don`t want anything to happen when I click the portal, It is simply a window to a single related record.
This portal fills the tab (which is most of the screen) and appears transparent to the user. However, when the user clicks in space, the portal becomes selected, ie grey, ready for duplicate, delete etc. All these functions are locked out so selection serves no purpose.
Cover the portal with a transparent rectanble set up as a button and select the "exit script" option. This should intercept the mouse clicks to keep the user from selecting an individual portal row.
Make sure that every pixel of the portal's portal row is covered by one field or another in the portal row. Select all of the fields while in layout mode and select the Commit records option to keep the portal row from being selectable.
In both options you can use appearance settings to prevent the button object from changing color when it is "pressed".