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My Dell ST2420 does this from time to time, Is yours permanently like that?
driver issue, re instal them
First time is done it and rebooting fixed it, it's about a week old.
Also only did it on one monitor, surely if it was a driver issue it would have been on both? Maybe it's the monitor?
I to have multiple monitors and only does it on one of them, I would send yours back, after you are sure its not a driver issue or graphics card fault.