In a post from 2014, I showed how to drive the sub-standard HDMI input of the Dell U2713HM 27″ UltraSharp at a resolution of 2560×1440 from the HDMI 1.3 output of a Linux-running laptop.
Fast forward 3 years, and I found myself having to drive the exact same monitor at its native resolution via its (sub-standard) HDMI input from a 2017 MacBook Pro through the brilliant HyperDrive USB-C dock.
(Apple, USB-C is nice, but you really pushed it too far this time.)
Although one can import Linux ModeLine timings into SwitchResX, the previous timings refused to work. It looks like this is due to macOS refusing to apply monitor timings which exceed the EDID-reported maximum pixel clock of 170 MHz.
Fortunately, SwitchResX is able to generate new timings for reduced blanking (this is crucial to be able to drive this monitor at its full resolution in spite of its HDMI port technically not supporting this) given the resolution and the refresh rate.
Setting the refresh rate to 42 yields a pixel clock of 162 MHz. This screenshot shows you how:
43Hz was probably also possible, but 42 is the answer to life the universe and everything, so there’s that.
To be able to setup these custom timings, you have to disable System Integrity Protection temporarily by booting into the recovery partition.