Display lines on 27-inch iMac may be Radeon Pro 5700XT GPU issue

A very small number of 27-inch iMac users are reporting issues with their Mac's graphics, with some encountering random screen glitches in the form of lines appearing on the display.

An example of the graphical glitch [via Apple Support user Bunn_pt]
In a post to Apple's Support Forums, users have documented the appearance of small lines on the screen of current-generation 27-inch iMacs. Images and video from one user reveal the glitches as being a single white line that spans the entire width of the desktop, one that briefly appears at seemingly random times.

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