Stale Scrollbar Client Crash

BUG ID: rdar://problem/5523503 Safari crashes clicking scroll bar in FaceBook 'Trips'

STEPS TO TEST: Drag the scroll thumb in each of the vertical scrollbars below.

TEST PASS: Each scrollbar will disappear when clicked, along with the box containing it, but the browser will not crash as you continue to drag.

TEST FAIL: The scrollbar will disappear and Safari will crash as you continue dragging.

OneTwoThreeFourFiveSixSevenEightNineTen