i can boot my iphone 8 plus screen on my iphone 7 plus with no issues, touch works great, no lag, no boot loop. but the screen is physically different. the phones also share call speaker and front camera assembly, allthough not sure the proximity sensor etc are the same internals but they plug up to the iphone 7 and 8 and you can take them from a 7 screen and transfer all parts o 8 physically Update (01/12/2019): just to conclude here, i changed screen, front camera flex and front speaker again, and it worked, i have a feeling it was either the screen blocking proximity sensor or a faulty earpiece speaker
i have n issue right now that a 7p boot loops after a screen replacement but if i unplug front camera flex it boots normally and stays on, if i plug in cam flex with phone on it works for 2 mins then starts boot looping, not sure this is any relevance to your issue but try running with front cam unplugged and see what happpens
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