MirrorDisplays error every 30 seconds in system.log
I had the same log complaints. They are the result of macOS detecting the secondary HDMI display and some code quality issue.
Uncheck "System Preference > Display > Show mirroring options in the menu bar when available." After reboot the log output will be gone.
Unchecking "Displays have separate Spaces" in System Preferences > Mission Control
and restarting the actual session will stop this useless error messages and useless Agent.
If "Display have separate Spaces" is already off, you will have to turn it on first to make the transition to off.
I was able to resolve this problem on my system by modifying the LaunchAgent file to only load in the LoginWindow.
In addition to eliminating the errors every 30 seconds in the log, it also appears to also have resolved an issue where my display would not turn off, regardless of the setting in Energy Saver. So, I'm pretty happy.
The steps I took were as follows:
-
Disable System Integrity Protection so you can edit the .plist file. Do this by rebooting into the recovery partition with cmd-R, open Terminal from the Utilities menu, and type:
csrutil disable -
Reboot into yoru system, then edit the .plist file with this command:
sudo vi /System/Library/LaunchAgents/com.apple.preference.displays.MirrorDisplays.plist -
Comment out the line that causes the MirrorDisplays tool to load. Change this line:
<string>Aqua</string>to this:
<!-- <string>Aqua</string> --> -
Then, reboot and reenable System Integrity Protection using the recovery partition as described above and type:
csrutil enable Reboot and enjoy a clean log file!
Editing the plist file as explained in Carl Hewitt's answer caused windows that extended across two monitors to only show on one of the two screens. The other portion of the window simply disappeared.
The following resolved the issue for me: Launch System Preferences, select Displays and uncheck "Show mirroring options in the menu bar when available."
System: 2017 27" iMac High Sierra 10.13.6 with two additional 27" ASUS displays connected via HDMI at the display. Credit to Allan & Bath Thank you.