When I run the demo widget for Tk4.0 and try to slide the scrollbar
I get:

  bad option "delta": must be activate, cget, configure, fraction, get,
  identify, or set
      while executing
  "$w delta [expr $x - $tkPriv(pressX)] [expr $y - $tkPriv(pressY)]"
      invoked from within
  "set delta [$w delta [expr $x - $tkPriv(pressX)] [expr $y -
  $tkPriv(pressY)]]..."
      (procedure "tkScrollDrag" line 7)
      invoked from within
  "tkScrollDrag .s 9 289"
    (command bound to event)

It makes perfect sense to me.

scrollbar.tcl has this:
  set delta [$w delta [expr $x - $tkPriv(pressX)] [expr $y - $tkPriv(pressY)]]

at around line 231 and $w must be a scrollbar widget. But what is the
code supposed to be?