Dirty screen fix with a dark frame

I got the following problem with my monitor (which is out of warranty already): its middle part is a bit brighter than the top and the bottom and if you present the screen as a three-striped flag tne brighter area is the exact middle stripe, a perfect rectangular and not a spot or a cloud. I consulted a specialist who said it has to do with power apllied to one of the three pixel driver circuits. The stripe is very vague and can only be seen on greyish and dark bluish surfaces. It is just a very little bit brighter and there's no problem with 2D, but in games it becomes destracting in dark areas especially while moving the camera and epsecially while doing this up and down. It's again like a dirty screen effect but wth the whole 1/3 of the screen. My question is logically: whether a shader can be written which decreases the brightness of this part but does it by a very little amount, again it's a perfect rectangular which is a middle stripe of a three-striped flag.

You could try using a shader that darkens the screen (like levels, curves, tonemap...) and than limit its effect just on the center of the screen by using a mask with the UIMask.fx shader.

Good to hear there is even a solution but not the one "go buy a new monitor" but before beginning digging in there's one more question: will this mess with UIMask.fx limit just one chosen shader or all of my shaders? 'cause I use deband as a must-have and don't want to loose it... limiting all shaders to a frame would be a bad trade

UIMask is split in 2 in-game effects: UIMask_Top and UIMask_Bottom.
To make this shader work you have to activate both of them and put your brightness-lowering shader between them. Any shader not located between the top and bottom ones will not be affected by the mask, so you can safely go ahead without any concern.

Already figured out that while experimenting in-game, a few moments before your answer))) Thanx for the tip Durante and for your sense of humor (in the spoiler) ...if only we always had the opportunity to buy everything we would probably loose the ability to think)))

Well... gave it a shot. No luck. No matter how smooth the gradient on png, in-game the effect leaves banding, hence the borders of the frame clearly seen in movement... Assuming the frame has smooth transition I think there will be much better result.
P.S. After some more attempts found out that LiftGammaGain.fx does the best. Still can't fully eliminate the borders left by the png template, very vague but still seen. I guess a separate shader has to be written to achieve gradual transitions.
P.P.S. Duplicated UIMask and LiftGammaGain, now with a second shader set made a new mask to hide the borders of the first one... having interesting results, very close to acceptable... will keep on tweaking