I am working on the SWT (Standard Widget Toolkit) for Eclipse.  We are 
trying to provide a common Java API for native widgets across several 
platforms.  The following information is available on Windows and Motif 
but I have not found the equivalent for GTK:

In the value_changed signal of GtkScrollbar (GtkVScrollbar or 
GtkHScrollbar) I need to be able to determine by what mechanism the 
scrollbar value has changed - i.e. was the thumb dragged (and if so is the 
thumb still being dragged) or was the up/down arrow button clicked (or 
keyboard up/down arrow) or did the user click on the space between the 
thumb and the arrow button (page up/page down).