We don't show the notification when the slider is adjusted in the popup menu, so i'm not sure it is necessary. We do show the notification when the mousewheel moves over the volume plugin, but as we don't hear a sound seeing its level makes sense in a notification, but for the brightness, we can see on the screen, so a notification doesn't really give any benefit. So WONTFIX for me.
There is an old bug report for this one: https://bugzilla.xfce.org/show_bug.cgi?id=13709
I still think it should be shown for consistency. Also if you only use the scroll function than you never know "where you at" brightness wise.
With the notification one can for example see that the screen brightness is at approximately 75 percent. Without it, one can only guess it from the actual screen brightness, which is really hard / not possible when the light in room changes over the day.
The notifications would be superfluous when scrolling - you already see the brightness value in the scale element that you are changing.
Also the notification bubble would either be above the plugin menu (making it impossible to properly change the scale element) or below the menu, which would make it impossible to read.
So the current is the correct and desired behavior.
So Simon gave his opinion in the old bug and its the same as mine, though a notification bubble above or below the plugin menu isn't what wasn't being asked for and the notification bubble through xfce4-notifyd is what is being asked.
So I tested other DEs and here are the results for the scrollwheel
Mate: power icon (doesn't work), sound icon (works without notification)
Cinnamon: power icon (works without notification), sound icon (works with notification)
KDE: power icon (works with notification), sound icon (works with notification)
@orschiro: when pressing a button on your keyboard, showing a notification is an additional confirmation to the user that the OS understands the key being pressed, which isn't the same when using the mouse. I was trying the brightness Fn keys on Windows and nothing happened at all, which confirmed that Windows didn't handle them.
@Frash: People don't judge brightness based on the percentage value, they judge it by looking at the screen. Similarly people judge the audio level based on how loud audio is coming out of the speaker. But though I disagree, adding it would make it consistent with the scrollwheel on the speaker icon.
So in this case I'd be in favor of showing the notification so it's a little more obvious to users what they're doing. Code-wise this should be a fairly simple change, so if anyone wants to dip their toes into power manager...
I think @orschiro and @Frash are right. Besides consistency (not only the volume icon behavior, but brightness keys as well), additional feedback (because why not if it's not bothering), clearer scale with the current value, there is also a case of remote work or assistance where it is useful for remote user to see what brightness is set locally.