Blurry text on DELL Monitor with MacBook Pro

This seems to be a common issue, and I had the same with a DELL P2418D QHD monitor.

It seems the best solution is not to buy a DELL monitor! But this worked for me.

Connected via HDMI the monitor was recognised as a TV and was in YPbPr mode. You cannot change this.

Connected via DisplayPort adapter the monitor was properly recognised and was in RGB mode. The text was still blurry.

The Mathew Inkson fix (https://www.mathewinkson.com/2013/03/force-rgb-mode-in-mac-os-x-to-fix-the-picture-quality-of-an-external-monitor) only ensured the monitor was in RBG mode when connected via HDMI.

I tried many of the various resolutions available via the RMD app (link below) but these made little difference. Text remained annoyingly fuzzy around the edges.

I finally hit on the solution by accident. 

The trick is to leave the lid up.

I had the MacBook Pros' lids down, so the monitor was the main display.

So this works for my Early 2015 13" Retina MacBook Pro and a Mid-2017 15″ MacBook Pro, connected to the display via TB2-DP and TB3-DP adapters respectively and with the displays mirrored. Both running Mojave 10.14.6.

1) Leave the MBPro screen up (but you can dim it to black)

2) In System Preferences | Displays, select the DELL P2418D (or your monitor) Panel, and choose Optimise for built-in Retina display

3) If you wish, then click Scaled. You can then choose one of 5 scaling options.

If you close the lid the external display will again become the main display, the “Optimise for built-in Retina display” option vanishes, and even if you then change the resolution to exactly match those in the scaling options seen in 3 above (e.g. by using the RMD app from https://github.com/avibrazil/RDM) the text still looks fuzzy.

I can't say the result is perfect, but it is a huge improvement.

BTW my LG 27UK850 4K monitor works perfectly with the above MacBook Pros and also a 2018 6-core Mac mini. 

Hoping that helps someone!