I just wanted to add to the above. I too had issues with white lines after replacing the battery in an iPhone 6. I followed the iFixit instructions, which involved disconnecting the screen - I’d not done that before. But the old battery was really stuck, and I needed the screen off to get the old one out without puncturing or damaging it (bad news!).
When I replaced the screen, at first it wouldn’t even boot/start up. It would just show a fuzzy image, and the board that all the cables attach to would heat up, then it would turn itself off. I tried reconnecting them carefully 3 times, and even made sure to get a little “click”, all to no avail.
I took a break, which is sometimes essential to clear the head, then came back and tried one last time. This time I used my fingernail to really press down on the back side of the digitizer cable. I heard more clicking, though I thought I might be tearing the cable too. With nothing left to lose, I continued across the back, and heard several more clicking/tearing sounds.
With much surprise, it then booted up, but had a block of big white bars about 2/3” from the top, and it also had smaller vertical bars all across. Progress!
Based on the post here, I wondered if somehow static was causing the residual bars. I left it overnight with the display on, and in the morning, the bars had faded to almost nothing.
So for anyone experiencing the bars, give it some time :)