I use Windows 7 and FileMaker 13.0v3 on a daily basis and have never seen such behavior for fields in a portal.
If you create a brand new file and set up the needed tables/relationships for your portal, do you see that behavior in this new file?
Have you tried changing the "in focus" appearance settings for the fields in your portal row?
Very strange right! I haven't seen this before either. It just started happening. No crashes of file. I have tried toggling the appearance settings. Its very strange. I have created a stand alone test file to mimic the functionality, but cannot get the test file to exhibit the same issue.
So I did go back to a backup form two days ago to a copy that did not exhibit this issue and one of the differences was I did not have a sort on the portal for the old version that was working correctly. I placed a sort on the portal and it immediately started misbehaving again.
Think the sort is causing it. I'm sorting on a number field that has a auto enter calc.
Does that mean that you can now create a test file that illustrates the issue or only that you can get your back up file to show the same behavior as your current copy?