This is consistent with the notification when the widget changes by
clicking. As far as I can tell, that notification was added shortly
before mouse wheel handling was added. It missing from the mouse
wheel handler was presumably just an oversight.
Otherwise, it will look like the value hasn't changed until the
widget is redrawn for other reasons, e.g. by mouse-over.
Incidentally, does anyone know why handleMouseDown() calls
sendCommand() when the selection changes, while handleMouseWheel()
does not?