Q: El Capitan unstable when connecting to external monitor using DisplayPort

I have the same issue with my 2015 MacBook Pro. I always have graphics card switching off so that doesn't solve it. It only seems to happen on my dell 4k monitor at home, my dell 23" at work has not shown the issue yet. It is very random for me.

I've had two issues with external monitors and El Capitan. I'm on a rMBP 15" mid 2014 with two graphics chipsets, the Intel Iris and the Nvidia, and the official 10.11.1 OS that came out yesterday.

I have a Thunderbolt DP to Dell 4k monitor as my primary external display, and not too much problem with that, except for the wake-up from sleep issue (fully described by me and a cast of thousands on an EC Beta thread that may no longer exist). That problem only seems to occur when the Automatic Graphics box is checked. The thing is, I keep unchecking the box, and it keeps checking itself again. I can't figure out why, or what to do about it.

The second issue--altogether new to me with the 10.11.1 release, is that if I plug in my second external monitor, a Dell 1080p monitor, HDMI to HDMI, the screen freezes completely on the MBP, Dell 4k, and Dell HDMI monitor, and I have to do a hard restart. I don't know if stuff is still running behind the scenes when that happens.

Needless to say have done all the SMC and PRAM resets, booted into Safe Mode, then back into regular mode. There must be some preferences corruption somewhere, but I don't know how to find it or correct it.

I Also updated to 10.11.1 and it did not resolve the issue for me, to add to it my location services also stop working when I connect to my dell 4K monitor. my experience so far is it only happens on my 4K monitor, not my regular 24" dell monitor at my office.

For me, it happens when connected to my 24" Dell U2412M.

And again, for when Apple starts to notice this thread, the problem requires a HARD RESET of the Macbook to recover, so it is a really bad problem.

My issue also happens to be with 24" Dell monitors. I've rolled back to Yosemite pending some sort of fix. 10.11.1 definitely DOES NOT fix it

That's correct, Giacecco. The U2412M is 1920x1200.

Yes, it only applies to DisplayPort connections and not HDMI, so far as I know. There have been all sorts of problems dating back to Yosemite, but more so in El Capitan, with crashes, wake up from sleep, freezing screen, etc. most of which leave the computer still running (but no ability to control it or see anything on the screen).

The resolution is definitely inferior with HDMI as compared to DP connections. For one thing, you can't run high resolution at 60 hz with HDMI, only 1080p--higher than that only runs at 30 hz.

I have reported this, as have, I'm sure, others, under the Apple Bug Reporting system, which is available to developers. No, I am not a developer, but Apple seems to think I am. Who knows, maybe I will be a developer someday. Say, when the Cubs win the World Series.

I have also only had it happen when in clamshell mode. Has anyone had it happen when the computer was already open and awake?

Indeed I have. In fact, I keep the lid open all the time now.

@giacecco, that is definitive. I didn't think HDMI was affected, but it is.

I wonder if this problem occurs with a new, clean install of El Capitan, without any migration from Time Machine from a Yosemite system. There may be some leftover drivers or kext files or plists from Yosemite.

I'm tempted to try a test clean 10.11.1 install, but that wouldn't identify the cause. Maybe I'll get a reply from my Bug report.

Here are the results of my "experiment" this afternoon.

I have a MacBookPro Retina 15" (mid 2014) with a Dell P2415Q monitor attached via TB-DP. No HDMI device is attached. The lid to the MBP is open.

A fresh install on an external SSD, USB 3.0, of El Capitan, updated it to 10.11.1. It is clean--no restore, no files brought over, nothing.

I disabled automatic graphics switching, and told the computer and hard drives not to go to sleep when the monitor went to sleep.

After a couple reboots, with everything seemingly stable, I let it sit for an hour. Both screens were off. I hit the Apple external trackpad (Bluetooth), and the MBP screen went on, but the Dell monitor screen did not go on, no matter what I did. The trackpad cursor was very slow and jumpy, not as responsive as usual, but the computer worked. I went to System Preferences, Display, and it showed both displayed, mirrored, as I had left them. Still nothing on the Dell screen. I then unplugged the power cord to the Dell monitor, plugged it back in again, and both screens were on, with the cursor back to its normal speed and state.

I'm not sure what conclusions to draw from this other than that it had nothing to do with "corruption" from old Yosemite installs or 3rd party apps, because there are none, and I had installed EC on a freshly formatted and empty drive. The fact that unplugging the power from the Dell (but leaving the DP connection intact) then plugging the Dell in again and having the display and computer work normally tells me that this is a malfunctioning EC driver-monitor connection of some sort, but that doesn't help to sort out exactly what it is.