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
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=== Update (01/12/2019) ===
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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 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