The shadow is because you have one of the two strips of LEDs that make up the backlight not working, and it is usually not working because it has an open line in this scenario. You create an open line on the backlight circuit either through damaging a pin at the connector, or connecting a screen without first disconnecting the battery which can blow the backlight filter, or if you have a water damaged induced short (or spontaneously occuring) on one of the two backlight circuits.[br]
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You’ve already ruled out bad screen which was another possibility, so you know this is a logic board repair. You can either buy all the equipment and learn how to do microsoldering and board level troubleshooting (iPhone 8p backlight is not bad for a beginner project), find a local microsolderer or use a mail-in logic board repair service.