Entering clamshell mode reduces the brightness of external screens unexpectedly when brightness syncing is on with syncing external changes is enabled and auto brightness is also enabled

[waydabber]

Mentioned in #2969 by @acocoadog:

If the cover of MacBook is closed when the brightness synchronization function is turned on, the brightness of the external screen will drop unexpectedly, but I think most people hope that the brightness will not change at this time.

[waydabber]

Hmm. I can't seem to be able to reproduce this issue. @acocoadog - can you give some more details on your setup and how syncing is configured exactly so I may try to reproduce this problem? Thank you!

[waydabber]

Ok, I was able to reproduce the problem. This happens only when auto brightness is enabled.

[waydabber]

It seems like when auto brightness is enabled and the system is entering clamshell mode, the system sends one brightness change notification about setting the brightness to zero. Strangely this does not happen when auto brightness is off. Added a check whether the display is in clamshell mode and prevented syncing when it is. This seems to fix the issue.

I tested/fixed this on an M3 Apple Silicon MBP, will need to see if Intel behaves similarly.

[waydabber]

let me know @acocoadog if you'd like to test an interim build with the fix to see if the issue is indeed resolved on your system as well.

[acocoadog]

let me know @acocoadog if you'd like to test an interim build with the fix to see if the issue is indeed resolved on your system as well.

Thank you for your efforts! I would like to help test it. I have M1pro Apple Silicon.

[waydabber]

Sure, I will - just found some issues affecting externally induced brightness syncing (some events are ommitted which causes issues with delta syncing #2973) I'll need to fix first and then provide a version.