Horizontal line on top of the display

"Same problem for me.

In my case if you have no Apple Care and still don’t have money to change it at the reduced cost.

Use external Screen, you may feel your MBP has become a Mac Mini but when you spend time like week or month then back to using the internal screen, this problem will disappear! I can use my system for 2 days and the line doesn’t comeback but I can feel that my systems screen still has the problem.Seeking for A.R. Coating issue on your screen. If you’re lucky enough to have this problem. Go to Apple Store and claim AR Coating Failure. Apple will change ALL screen parts for you, and yes it free!"

"did you turned off the Mac or just the screen ?

for me I use an external monitor and I use at the same time SwitchResX to turn off mac's display but when I turn on Mac's screen (retina) I notice the same problem even if all the time I use external monitor ....

Update (05/22/2020)

After using the bars becomes a Wall !!"

Does replacing the display solve the problem ?