I suggest you let it go. I went to the Genius Bar at a large Apple Store in Italy last Saturday to have a system diagnostic done because of a reboot issue.
Among the various tests, I saw a “touchscreen test” listed. Well, they ran it, and the test was simply sliding a finger over the screen to color blue squares. It’s a test that any screen test app can do (even Aida, if I remember correctly).
Something like that:
View attachment 2444292
If you go, let us know if they perform a more in-depth test for you.