Chosen Solution
After replacing the touch screen I find that in messages, the return button at upper left corner is sometimes inactive. usually, closing the app with home button and re-entering the app fixes it. I had several other similar experiences in other apps. is this hardware or software?
I would lean towards a hardware issue. In particular the screen assembly . I have found in the past that the iPhone 6 Plus replacement have touch sensitivity issues due to the LCD ribbon cables not being fully seated in their respective socket. I would remove the LCD and reseat their ribbon cable terminals into the socket.
