I've been enjoying Dell's 25" UltraSharp U2515H display for the past few months. It has excellent color accuracy, a bright backlight, and great resolution for the price.
I’ve noticed one oddity though, the display produces a strange dithering effect on shadows and blurs in macOS Sierra.
The issue seems to be in the display's presentation of the image, and not the image itself. Screenshots captured from the OS, like the one below, don't show the dithering.
It took me a week before I could pin down the issue. It appears to be a bug with either the display or my old Mac mini's handling of DisplayPort 1.2. The fix is simple enough, disable DisplayPort 1.2 in the monitor's settings menu and the blurs and shadows show as expected.
To disable this setting, tap on the menu button (third from the left) to bring up the display’s configuration menu.
Next, navigate down to the Display section on the left hand side using the down-arrow (second button from the right). Then use the right-arrow (third-button from the right) to navigate to the right panel. Select the DP 1.2 setting using the right-arrow and set it to Disable and confirm.
Now blurs and shadows render as you would expect. No more weird dithering.
I can't remember if this is something I turned on out of stupidity or if the display comes from the factory with this setting.
Either way, I hope this can save someone else the headache.