Click on "bar" part of a slider - suppressing value jump

[Jim Witte]

   I've got a slider implemented with a scrollbarDrag handler and a 
mouseup handler so I can option-click on the bar to exactly set the 
value.  If I option-click on the thumb of the slider, it doesn't change 
the value before executing the mouseup script as I expect.  But if I 
click on the bar part of the slider, the thumb flips to one end or the 
other before the mouseup handler is executed.  The range of the slider 
is only 0 to 800.

   Is there any way to supress the "scroll to" behavior of a click in 
the bar area before the handler executes, or should I set up the 
mouseup handler in an enclosing group?

Jim