I have the same borders on my third monitor connected via dp-vga but only in a non-native resolution like 4800x900, but it's fine while 5760x1080. Can you make a photo of these borders? I want to compare with mine. This bugging me a lot, not sure what to do also
I also had these borders on another system with the same GPU+Monitors. Back then I had another version of the CCC where the overscan option was not greyed out. I could drag the overscan slider to maximum and the borders vanished. Strange thing.....
(Photo is coming. Not at home right now.)