I've tried this out on a database of my own, which has a similar portal. The answer is, it's because you have 'allow data entry' unchecked. This automatically seems to give the black highlight of the whole portal row, which you don't get with data entry allowed. Probably because FM regards the whole row as a single, unmodifiable display item.
If you allowed data entry, there are ways you could protect the data to some extent, :
1. Change the portal format so that "Allow deletion of portal records" is unchecked
2. Change the relationship definition to ensure "Allow creation of related records" is also unchecked
3. Create a new value list with a single value "Do not modify this field", and apply it in the Field Format to each field in the row. That won't prevent anyone from actually changing the content, but it at least provides a visible warning.
If there is any other way around this, I haven't discovered it yet.
Going to the post that Sorbsbuster posted, the idea of a "do nothing button" sounds ideal - but, why not place it in front of the other fields in the portal and make it transparent? After all, your fields are display only.
Which does lead to another question - why are your users clicking to select a field when it's only for display purposes? After all, they can't modify anything.
When the user clicks on a field, there is actually a transparent button on top to run a script to set up variables containing the location of the cell, bsaed on which different processing will occur.
By the way I tried all solutions suggested and I am still struggling.
I even have another problem: when I remove the button and click on the field directly I get a wierd error, it says "No records are present. To create a new record choose the new record menu command"
If you want to see the file I am working with (fp7) I can send it but I do not know how to dend it.
After many trials I found out that I have a script trigerred by onlayoutenter to go to a certain portal row and when I disabled it the highlighting problem went away. I do not understand why but it solved my problem. It would be interesting to understand how file maker works in that respect.
A row gets highlighted in a portal to show that it is the currently selected record. Where this becomes important is when a user selects Delete Record from the Records menu. If a portal row is highlighted and portal row deletion is enabled for that portal, the highlighted portal row's record will be deleted. If it is not, the Layout's current record will be deleted. The Go To Portal Row script step, if "select entire contents" is selected, selects the portal row and thus it will be highlighted. If you clear that option, this script step will not highlight the portal row.