Thin red line on screen
How do I get rid of a single red line across my iPad screen
iPad Air, iOS 12
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A reliable method to confirm a screen or other hardware fault is to take a screenshot when the fault is evident - and then review the captured screenshot image...
When reviewed, if the lines are not present in the screenshot image, then you have demonstrated that you have a hardware fault that, sooner or later, may become sufficiently severe that repair or replacement of the iPad are the only options. If the lines are visible in the screenshot, the something other than hardware will be the cause.
Note: When reviewing the image, rotating the iPad screen orientation by 90°, from Portrait to Landscape, will ensure that the image does not coincide with screen defects if present.
Take a screenshot on your iPad – Apple Support