Comment 4 for bug 1389761

So, here's the plan.

We'll add a "system volume slider" as a standard toolkit widget that any app can use. This slider will do three special things:
(a) use standard volume icons by default;
(b) automatically reflect, and adjust, the system volume for the active output role, without the app having to do anything at all;
(c) suppress volume change notifications (but not primary sound output change notifications) whenever it is present in the active window. (But not, for example, when it is in a window behind the active window.)

The sound menu should use this slider, fixing the problem that redundant volume notifications come up with the current slider.

System Settings should use this slider, fixing the problem that redundant volume notifications come up with the current System Settings slider.

And any music player, for example the Music app, should use this slider too. An app should be able to customize its appearance, while still getting its behavior for free.

One drawback of this plan is that it won't fix bug 652978: it won't suppress notifications for track changes.