ScrollBarSlider

The ScrollBarSlider is dual-purpose: drag it to scroll the contents of a 
ScrolledWindow and read it to identify the portion of the window's contents you 
are viewing.

Mouse down and drag on the ScrollBarSlider with either the first or second mouse
button to scroll the ScrolledWindow. The contents scroll in the direction that 
you drag the ScrollBarSlider. The contents scroll as you move the slider until 
you release the mouse button.

As a feedback mechanism, the ScrollBarSlider provides two helpful types of 
information. First, the position of the slider inside the ScrollBarTrough 
indicates the position of the portion you are currently viewing relative to the 
length of the complete contents. For example, given a vertical ScrollBar, when 
the ScrollBarSlider is positioned at the top of the ScrollBarTrough, you are 
viewing only the topmost portion of the window's contents. If you scroll so 
that the ScrollBarSlider moves to the middle of the ScrollBarTrough, you are 
viewing a portion at the middle of the window's contents.

The other feedback property of the ScrollBarSlider is that its size changes 
depending on what fraction of the entire contents is currently visible in the 
ScrolledWindow. To see this feature in action, resize the parent window of a 
ScrolledList that is only showing a fraction of its contents. If you resize the
ScrolledWindow larger, the fraction of the visible contents grows and 
consequently so does the size of the ScrollBarSlider. When the ScrollBarSlider 
takes up the entire space of the ScrollBarTrough, you are seeing the complete 
contents and cannot scroll the ScrolledWindow in any direction.

See also: ScrollBar, ScrollBarArrow, ScrollBarTrough, ScrolledWindow
