Un-lock frozen System Settings Display function

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Level 1

34 points

Un-lock frozen System Settings Display function

2019 Intel iMac: After installing OS 15.7.9, I cannot change the (Display) settings in System Settings. The secondary monitor (a Cinema display) now acts as the primary unit, and the Built-in display is a mirror of the smaller Cinema display.

When I remove com.apple.windowserver.displays.plist with Terminal the set-up returns to normal, until I reboot or turn off the iMac. I have re-installed the system (Sequoia 15.7.9) but the problem with the Frozen/locked display settings remains.

Any suggestions/help would be appreciated

iMac 27″, macOS 15.7

Posted on Sep 3, 2026 6:09 PM

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Answer 1

Top-ranking reply

WheelieNick

Level 9

59,924 points

Sep 4, 2026 12:31 AM in response to Old Art

The reason your display settings keep locking up after a reboot is that macOS Sequoia (15.7.9) is regenerating a corrupted system-level display configuration cache every time it builds a fresh connection to your Cinema display.

Because deleting com.apple.windowserver.displays.plist fixes the issue temporarily, the underlying problem is likely a combination of a persistent NVRAM/SMC bug caching the old display configuration layout and a system-level directory permission mismatch that prevents macOS from permanently writing your new display setup.

Since you are on a 2019 Intel iMac, your display layouts, primary monitor assignments, and port data are stored deep within the hardware NVRAM and SMC. If these are corrupted, they will force the secondary display to act as the primary every time you reboot.

Reset the NVRAM: Turn on your iMac and immediately press and hold Option + Command + P + R. Keep holding them for about 20 seconds until you hear the startup chime a second time or see the Apple logo appear and disappear twice.
Reset the SMC: Unplug the power cord from your iMac. Wait 15 seconds. Plug the power cord back in. Wait 5 seconds, and then press the power button to turn it back on.

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Answer 2

den.thed

Level 10

97,268 points

Sep 5, 2026 11:40 AM in response to Old Art

You need to point the arrow notch at the iMac Display and select Use as ________ Main Display.

Then point the arrow notch at the Cinema Display and select Use as ________ Extended Display.

Please ignore the other information on those screenshots.

They are from a Mac mini running dual displays, but the Main and Extended Display options work the same.

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Answer 3

Old Art Author

Level 1

34 points

Sep 4, 2026 7:21 AM in response to WheelieNick

Excellent advice, I have reset the NVRAM & SMC as instructed all is back to normal.

Thank You very much

This old head never thought of the NVRAM, didn't have one my old Apple II+. (I think!)

I will keep a copy of your instructions, just in case!

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Answer 4

Old Art Author

Level 1

34 points

Sep 5, 2026 9:48 AM in response to WheelieNick

WheelieNick

Well, I'm back, need your advice again.

When I booted up again today, the problem has returned, the secondary display is acting as primary, the built in display set to mirror, and the display function is frozen.

I've redone using the terminal to remove the plist with;

rm ~/Library/Preferences/ByHost/com.apple.windowserver.*.plist

sudo rm /Library/Preferences/com.apple.windowserver.plist

and about to reset the NVRAM and SMC, just in case I screwed up the settings yesterday.

I'm wondering if I should try re-installing the previous version of the system prior to 15.7.9.

Has anyone else reported any problems with this update on an older Intel iMac?

I have to run out right now, but I would greatly appreciate any suggestions. If i ran EtreCheckPro 2 v6•8•15 and sent you the report do you think that might help?

Anyway, I do appreciate your help

Old Art

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