Switching resolution loses main display setting

[plangin]

When switching between resolutions, the dummy screen switches from 'main display' to 'extend display' by itself.

My setup:

• MacBook Air M1
• macOS 12.0.1 Monterey
• LG 34WK95U-W

[waydabber]

This should not be the case. Can you show me the screenshot of the Displays tab in System Preferences where you do the resolution change so I might quickly overview if settings are correct + make a screenshot after the change when the mirror breaks down? If you could give the overview screenshot (where you see the screens) that would be useful as well. Thank you!

[plangin]

Thank you for your quick answer!

This is how it looks before:

And after:

Additionally, the screen arrangement does also break. But I assume that is a macOS thing.

[plangin]

Ah, the overview:

[waydabber]

Nice. What will be the Main display after changing the resolution? The MacBook display (that should preserve the mirror but only relegate it to an extended mirror set) or the LG HDR 5K (in which case the mirror set breaks?)

[plangin]

Afterwards:

MacBook Air: Main display
LG HDR 5K: Mirroring Dummy
Dummy: Extended Display.

[waydabber]

Hmm. It seems like the mirror remains intact but for some reason macOS relegates the mirror set to Externded. This is not a problem if you use your MBA in clamshell mode, but if you leave it open, then probably during configuring the displays as the Dummy is unavailable for a while the Displays app assigns a Main role to the MBA display which is not in a mirror set and then forgets to reassign it to the updated mirror set.

I probably cannot fix this one easily as this seems to be a macOS issue. I plan to have the option to manage the display configuration programmatically inside the app (but I'll also leave the option to let the user do it the way it is now), I can watch out for this scenario then.

[waydabber]

(I converted this one into a discussion as there is nothing much to do about the issue as of now, but others may benefit from the info found here.)

[plangin]

That's absolute OK for me.

Personally, I won't switch the resolution that often and like written in my 1st answer, I was already afraid that it is a normal macOS behaviour.

[plangin]

Strange, now it works as intended:

I can step through the resolutions and the external screen stays the main display.

What I did this time (between many reboots): Erased old unneeded color profiles from past monitors and Dummies in /Library/ColorSync/Profiles/Displays

[waydabber]

Yes, every time you create a new dummy, a new color profile is created for it and also windowpreferences get littered with additional display definitions. That's why it is best practice to reuse Dummies instead of discarding and recreating them.