This is classic theme:
http://3.bp.blogspot.com/-zqGiJCBubZI/Td51yumWL_I/...
This is modern/aero theme:
http://4.bp.blogspot.com/--VAFXW8L0IA/Td51yN6qm0I/...

Windows 7, classic theme?

What is "the first pixel of the button"?
How is your question related to the tweaker?

The "multipage_wheel_scroll" option, as well as other scroll options, works globally regardless of the focused window. Ctrl+Alt+Tab uses a regular window, and mouse scroll events get stolen from it.
Nevertheless, I agree that when the taskbar switcher is open, you are probably messing with it and not with the taskbar. There shouldn't be a problem to whitelist it.

Same as this?
http://tweaker.userecho.com/topic/352629

Do you have the "multipage_wheel_scroll" advanced option enabled?
Or any other mouse scroll options?

I can't reproduce it. Also, the recent update didn't introduce changes regarding the volume control.
Perhaps you're using the "Cycle between taskbar buttons" option or the "multipage_wheel_scroll" advanced option.

The most sane desktop manager for Windows I know is Sysinternals
Desktops. The main problem with it is that you can't move windows
between desktops, but all in all it's pretty stable.

I believe that what you're seeing is a so-called "Temporary notification source".
http://msdn.microsoft.com/en-us/library/windows/de...
The temporary icon is by default more distant than the other icons.