I think you are understanding my question just fine. I read that page and double checked everything, restarted everything etc etc. I thank you for that, although it didn't change anything. Universal control itself is working like gangbusters except for this one thing. On the iMac : In Display settings : Select the MBP : In the drop down menu for "Use As" you can select "Use as mirrored or extended display" and then that works, i am mirroring the iMac screen to the MBP On the MBP follow same steps: only option for the iMac is to use as Linked Keyboard and mouse. The option to use as Main Display or mirror or extend isn't there.

Well... if I had a studio display I would be using it with the MBP, not the 2015 Mac. Just the same, yes, they seem to want to cause us to believe that it's got a hardware deficiency somewhere. However, I don't believe it: aftermarket products make it work! Luna Display promises this function. Just not quite at full pop. It'll do 5k @ 45htz instead of 60. But will do 4k at 60... I find it so very hard to believe that we can repurpose 30 year old satellites that are speeding around at thousands of miles per second millions of miles away to do things they were never intended to do... but my old desktop monitor is obsolete. This is most likely Apple style marketing (but I really hope its not, and hope they are done with childish things like intentionally killing battery life to make you upgrade phones). Waiting patiently to see if they intend for this to work and I am doing something wrong or if, since still in BETA there is a glitch somewhere. Yes, I could sell the old iMac and drop more coin on the Studio Display, but the old iMac won't fetch much I suspect, and I'll still be out another $1,000.