I have a "stock" refurbished HP Compaq Elite 8300 SFF with i7-3770 processor. Video has not been a problem driven from the onboard Displayport using a DP to HDMI cable - until I try to run a display with more than 1920 pixel width. Using an LG 29wk500 21:9 the display image is broken into repeating, overlapping "screens", complete with a pointer for each - very confusing to try to navigate while searching for a solution.
I have the computer set up as dual boot with Windows 10. When booted from Windows 10 the display works perfectly at its native resolution without intervention in Windows settings.
To see if the problem was just with the Mac OS, I installed a PCI graphics card recommended for this computer as a hack. With that card installed I was able to achieve full native resolution on the display using SwitchResX, as Mojave doesn't support the ultrawide display in its default selections. I applied a "safe" setting for the display's native resolution and it worked on a reboot, finding the proper resolution without intervention.
I shut down, disconnected the video and took the video card out, plugging back into the display using the displayport and the display ignores the settings for SwitchResX and defaults to the repeating, overlapping "screens".
Has anyone else experienced this and if so, were you able to fix it? It seems logical that if Windows 10 can easily use the proper resolution using the onboard video from displayport, and SwitchResX makes it work on OS Mojave using a PCI video card, that there should be a way to have Mac OS Mojave work with the onboard video.