I've had this issue with MC 7.0.3 - Mavericks up through the current MC 8.3 - Yosemite, on a 2013 "trashcan" Mac Pro. Trying to connect 2x Thunderbolt displays and a TV via HDMI, in that arrangement. Full Screen Playback, when set to my third display, always sets its values to 0-0-0-0. Then enabling Full Screen Playback crashes the application.

Full Screen works with any combination of two displays, on the TV connected with a Thunderbolt adapter and even with all three connected, as long as Full Screen Playback isn't set to the third monitor in the Display Arrangement. Enabling each Display as a Space didn't help either. Finally I realized a good workaround is to set my client TV as the first monitor, with the toolbar. This makes Full Screen Playback work.

But it also means the issue is probably unrelated to the HDMI connection - it's either the OS or it's Avid.

Since I googled the heck out of this setup in the first place but never found any reference to this issue until I knew its specifics, if anybody else was planning to get a Mac Pro to run a classic three-monitor setup out of the box (Apple specs the system for SIX thunderbolts and three 4K displays), I thought it was worth posting this warning / workaround.

For obvious reasons this workaround isn't ideal. Does MC officially support Full Screen Playback on third monitors? This or a similar issue even seems to pre-date the new Mac Pros*. If we can't expect a fix anytime soon, can anybody vouch for MC 8.3 compatability with the Blackmagic Mini Monitor? I've had more positive experiences with Aja, but their equivalent is twice the price, I see other posts about their products bugging out lately, and I hate buying hardware to work around software.

Thanks.

*http://c2.avid.com/forums/p/117488/710234.aspx, http://community.avid.com/forums/p/123867/724609.aspx

I can't speak to your specific issue, but enabling full-screen playback in Avid 8.3 on my 5k imac (yosemite) always crashes the application the first time I try. I always have to crash it once, re-open and then it works...for the next few hours, until I have to repeat.
It's ridiculous. 

I would love to hear any resolution on this or know if it's been escalated to "bug status" or whatever, as I'm wanting to try a similar setup.... (8.3 or 8.4 on a retina 5k iMac osx 10.10.5)

Hey guys, so I found a solution. I have an iMac 5k 2015, a toshiba 25" as a second monitor and a BenQ projector for my client monitor. The way to get it to work is avid likes to play with everything being the same (which has been frustration for those coming form FCP or Premiere) and that includes monitors. So I have both my projector and Toshiba monitor set at 1080p and my 5k retina display set at one to the left from the default (closer to the Larger Text option) under the resolution option to set to "Scaled". It's still a little bigger than 1080p but when I open the Full Screen Playback out of my Avid settings and place it on my project and hit "Select Monitor" I finally get some numbers pop up under "Current monitor position:". Once they do, hit OK and you are in business. Obviously sucks because you are not utilizing the full resolution of the 5k retina display, but you can always switch it back and forth depending when you are working with clients.

Forgot the I/O boxes and out of sync playback, go straight out of your system and you are set. Hope this helps ya'll

Just be aware that computer monitors and tv monitors handle levels and scanning diffreently.  Don't try this with interlaced material and if you are going to use full screen display, check the box that scales video levels to full range for computer displays.