label {The left slider has changed( FL_WHEN_CHANGED | FL_WHEN_RELEASE | FL_WHEN_NOT_CHANGED) so it produces a callback on both drag and release mouse events.
The middle slider (representing a widget with low overhead) is changed on every mouse movement.
The right slider (representing a widget with high overhead) is only updated when the mouse is released, by checking if Fl::pushed() is zero.}