[Q] only top of touchscreen works topic

I had earlier reported this as a bad digitizer, but now not so sure. I'm hoping some of the experts here can advise me. I have an S4 (M919) in which everything works fine except only about the top 10% of the screen will respond to my touch. The display looks fine and I can control the top band (notification area, etc), but it doesn't respond anywhere else I touch. I swapped a different motherboard from another S4 and the same problem, so don't think it is a software issue.

What could cause this? Is it possible to replace just the touchscreen without having to replace the entire LCD which is at least 0 - 0.

Thanks in advance.