Sadly, this looks like a LCD panel defect. we call it a TABS error.
Think of it this way you have pad of graph paper which you tear-off a sheet and you want to tape the it back but you want the lines to line up between the sheets. Not easy! Your display is the same a flexible ribbon cable is glued to the edge of the panel to carry the signals to the panel in both rows and columns, just like a spreadsheet!
What happens is one of these lines has lifted up or was not aligned correctly from the start. Another cause is damaging the display in the process of taking it off because they used the wrong tools and technique.
You’ll need a new display assembly to correct it
Here’s the needed part 27” iMac Display Panel & Front Glass Assembly, Apple 661-7169 (Used Grade-B) The displays are getting hard to find! Here’s the guide Sostituzione schermo iMac Intel 27" EMC 2546. Make sure you get this kit [prodotto collegato mancante o disabilitato: IF174-005-1] which has the needed cutter and you should get two sets of Plastic Cards to help hold the areas you’ve cut away.
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