Chosen Solution
I followed the instructions from iFixit and used their tools and battery to replace my iphone 6splus battery and now there is a shadow in the upper left hand of the screen and all down the left side. I’ll try replugging the connectors if that’s the recommendation but I don’t want to make it worse!
I fixed it by opening the phone, unplugging the battery, unplugging the Screen connector as described in the guide for replacing the battery. then I plugged the screen connector back in, making extra sure it went in straight and I think it actually clicked into place this time, then screwing the cover for those connectors back on, then plugged in the battery and its cover and closing the phone up again. I didn’t replace the seal/adhesive though so it’ll probably fill up with pocket sweat and ruin the thing. any way it’s working again now - shadow gone.
