Dell U2415, 2018 MacBook Pro issues.

I have a new 2018 MacBook. Mojave 10.14.1. The built-in Retina display is working fine and looks wonderful. However, I am getting fuzzy and unfocused text and graphics from my external Dell UltraSharp U2415 monitor. The text is barely legible, images are not sharp and the colors lack depth (a bit like what you see when you modify an image from 16 bit to 8 bit, only more pronounced). From the USB-C port on the computer, the display is connected via a Display Port port . System Preferences reads the name of the monitor correctly and claims it is running at the “Default for display” resolution. Scaling it manually to the monitor’s native 1920x1200 resolution does not change anything. It says the monitor is running at 59.88Hz, which seems slightly odd to me but probably OK (I expected 60Hz). The colors are themselves accurate and adjusting the color profile does not help. Neither does playing with any of the settings on the monitor itself. The monitor is set to DP1.2 disable. Switching the cable and connection to an HDMI to HDMI does not make any difference. The monitor then switches the color input mode into YPbPr mode rather than RGB mode, which seems odd to me. With the Display Port cable connection, System Report > Graphics/Displays states the correct name of the display, resolution = 1920x1200, 32-Bit Color (ARGB8888) and the correct Display Port connection type. Resetting the NVRAM does not make any difference. I reconnected the monitor to my old White MacBook 2012 running High Sierra and it works fine, nicely sharp (connection via thunderbold to DP port, this time the monitor reports the color input mode as RGB). Connecting the monitor to a Windows 10 machine using its Thunderbolt 2 port and the MiniDP to DP cable, and the monitor works fine. It looks to me as if the graphics controllers of OS Mojave are not driving the U2415 display properly. I bought the computer for the primary purpose of photographic editing, which I had been doing using the U2415 quite happily on the old machine for over a year. The new MacBook Pro in its current state is not fit for that purpose, indeed I have had to take it out of commission until this problem is resolved.