Display split in half with lower resolution

G'day,


My iMac has a weird split-screen display problem when I use the "Settings -> Displays" to select "Scaled -> largest text". I am not using an external display, this is a problem with the iMac monitor itself. The screen appears to be split in half with the RHS correctly scaled to fill the right hand side of the screen but the LHS seems to not be scaled and instead have underscan bars inserted. 


If I use the settings to try and scale to higher resolutions (to smaller text) the scaling works fine.

I have tried resolution changes using the settings app but also using SwitchResX both with the same result. With SwichResX I was able to see the upscaling works fine to higher settings but downscaling to resolutions below 1920x1080 HiDPI, or indeed any non-HiDPI settings below 3360 x 1890 causes the same artifact. 


My system is an iMac (Retina 5K, 27-inch, Late 2015), with AMD Radeon R9 M395X 4096 MB, 4 GHz Intel Core i7, 32 GB1867 MHz DDR3 RAM).

I don't know if this has been a problem with the mac from the very beginning because up until recently I have had no reason to change the resolution.

Could someone tell me if this is a hardware problem or a driver issue? Is there anything I can do to fix it?

Many thanks in advance for all your help!


Cheers,


Rich