Casperes- you may be onto something. I just noticed the cable says full HD which I understand is lower than 2560 x 1440?
Some terminology:
HD Ready: 1280x720 (720p)
FullHD: 1920x1080 (1080p)
QHD: 2560x1440 (1440p)
UHD: 3840x2160 (2160p)
These sometimes go by other, incorrect names, like 4k and 2.5k. The terms are also sometimes used for very similar resolutions, like 1920x1200 sometimes being called FullHD, even though the FullHD spec technically only specifies 16:9, whereas the 1920x1200 resolution is 16:10. As far as I know anyway - the spec may be bigger than I know actually. Regardless, yes; 2560x1440 is more than FullHD.
I did read somewhere the xobox will only offer 1440 resolution if auto-detected by a compatable monitor which the iMac may not be.
So the way auto-detection of resolutions and other monitor details works, is a bit tricky. The reason again being that different specs have different methods of doing this, and the iMac does inform Display Port clients about itself, using the Display Port standard (EDID - Extended Display Identification Data). Newer DisplayPort standards also support another information table, known as DisplayID. I don't know what HDMI uses to transfer its data stream for a monitor's capabilities. EDID is a standard, so it may very well be that it just uses that too, but it's a standard made by the VESA group, who also are behind DisplayPort, so the group behind HDMI may have their own separate solution. An adapter may be able to convert this data stream as well, if it is not able to just carry it on. It does seem like this would be necessary to get 1440p though. Even then though, Display Port through an adapter should at least give you 1080p no issues, since I assume that's what the Xbox One defaults to if it gets no monitor capability details.
converting display port into hdmi is trivial.
converting hdmi into display port is hard.
directionality matters.
https://www.macworld.com/article/1151256/kanexxd.html
I'm only really quoting this to emphasise it. Display Port is made to be compatible with other standards, but how trivial the conversion is, isn't a concern for the VESA - just that it's doable.
[doublepost=1527023962][/doublepost]Just something you might want to read about EDID
https://www.extron.com/company/article.aspx?id=uedid&version=print
As well as a product guaranteeing to send the EDID information
https://www.extron.com/product/product.aspx?id=hdp1014k&s=4#desc
It doesn't sound like it's something of an expected standard that adapters will send the EDID data though. They say that their adapter asks the display about it's EDID data, stores it inside the adapter, and sends the signal to the source on power-up, making it sound like it's not passed through the cables, but has to be actively passed through the adapter.
