The standard behavior in a multi-display setup is that Photoshop uses whatever display profile the OS offers up for that display - until the image window crosses the halfway line between them. Then it switches to use the other profile.
The shift-on-drop thing indicates a video card problem. That's how OpenGL works in Photoshop, when you drag an image OpenGL is temporarily suspended, then snaps back on when you drop.
So I'd try to disable GPU in PS Preferences, or set it to Basic mode. This shifts all display color management back to the CPU. Then wait for the next video driver update from Apple.
BTW don't mess around with Photoshop's color settings - that's not where the problem is and it gets you nowhere (except in trouble).
This is about the display profile, not the document profile. The color settings dialog is only for managing document profiles.
Thanks for taking the time to reply to me.
I disabled GPU, and I think that solved the problem. Images on the NEC using various applications including PS now appear to match.
I do have one additional question. What exactly does disabling GPU do?
The GPU, when enabled, takes over some of the number crunching from the CPU. So it partly accelerates some screen rendering operations, partly adds some additional functions. None of these two are essential for Photoshop's basic operation and you can do without them - as indeed we all did prior to CS4 when it was introduced.
The important aspect here is that the display color management chain is handled by the GPU, when it is set to Normal or Advanced modes. In other words it performs the standard document profile to monitor profile conversion, and feeds the result to the display. The problem with that is that it's highly inaccurate and very prone to bugs and general errors. A buggy video driver can cause problems no end here.
I've had it set to Basic for years. With this setting you still have some of the OpenGL functions, but color management is handled entirely the traditional way, in the CPU.
Thanks for another thoughtful and informative reply. I did a little search on the GPU function last night, and with your help, I think I understand what's happening. Most important, your support solved the problem I was having. Unfortunately then, I ordered the MacBook with the NVIDIA GeForce GT 750M at extra cost thinking it would speed up rendering, but now have to basically turn it off when using Photoshop. You live and you learn.
On a note, fooling around with it and almost by accident, I found that if I open a file with the GPU checkbox unchecked, the file loads and is nicely converted to the NEC. But if I again check the GPU box back on AFTER a file is loaded and moved to the NEC, it does NOT effect or change the color. So I suppose I could use it with that little workaround. But I think I'll go with your kind and thoughtful advice. Apple only supports so much and the rest is up to the user, so a buggy bit of software/hardware is the last thing a color management neophyte like me needs to deal with.
As an aside, I have NEVER been able to get my MacBook Retina to calibrate properly anyway using an X-Rite calibrator and one the gamma testing screen image on this site as a crude visual reference Gamma calibration - Lagom LCD test I had an older non-retina 17" MacBook and it calibrated pretty darn close. But even after X-Rite calibration, the Retina is so far off the bars on the 48% luminance levels are off the chart to the top. So the MacBook is only useful for simple photographic operations, but completely unusable for RAW conversions or editing. That's the reason for the NEC, where both before and after calibration, the visual bars on the screen gamma image all sit perfectly at the gamma I calibrated for.
I hope I'm on the right track here.
Once again, THANK you for your timely and informative replays. VERY much appreciated!