Michael Natterer authored Jan 24, 2012 and Matthias Clasen committed Mar 01, 2012

If delta_x/y information is provided in scroll events, use it
to modify the underlying adjustment in steps proportional to
the deltas provided.

If the child widget of a scrolledwindow doesn't set
GDK_SMOOTH_SCROLL_MASK, regular scroll events will be dispatched,
and still handled by these 2 widgets.