That comment seems to suggest the opposite though ( @nclarius)? The X11 behavior is unexpected, because it changes only the internal display even when scrolling on the external monitor.
Yes, by that comment I meant that having only the laptop screen changed even when done so from a different monitor is unexpected.
But I did also point out the limitations of showing brightness information to be an issue for multi-screen adjustments in a different comment, which is what I think Jacob was referring to.
My concerns boil down to the interaction between the where changes are triggered by the user, where change information is shown and about which screen it is, and on which screen(s) the changes take effect, which so far seems somewhat confusing on X11.
So I think
Perhaps for 6.2 we can migrate the OSD to per-display APIs to remove OSDs from this discussion
would be a good compromise for the time being.
I agree that
Controlling all screens is better than some subset only.
If we can make this work I think this would be the ideal solution, but my understanding was that this is not really possible?
I wouldn't say that
the distinction between built in and external displays is completely arbitrary
I am more likely to be wanting to change the brightness of my laptop screen because I may use it in different lighting conditions, or because I want so save battery power - though that would then typically mean I'm not in a multi-monitor set-up anyway. Still, I wouldn't say it is completely arbitrary conceptually. If we do have to restrict ourselves to some subset then I think internal vs external screens are the most transparent distinction from a user perspective. But that's not the only choice we have, is it?
I'm with the arguments Xaver gave in his comments that it wouldn't be a net negative and in general don't tend to want to keep things the way they've been just for the sake of it even when they were not good to begin with, but I don't have a strong opinion on it if you, being the one who put in all the work, think that keeping the old behavior is the more feasible action plan for the near future.