Thumb is not updating on NQ of Scroll event, or on WS of Thumb

[mkromberg]

Run Demo_Issue296. Click on the "Goto Row 500" button. Note that this sends:

{"NQ":{"Event":"Scroll","ID":"F1.UPDOWN","Info":[0,500],"NoCallback":0}}
the client correctly responds:
{"Event":{"EventName":"Scroll","ID":"F1.UPDOWN","Info":[0,500]}}

The grid contents are updated, but the displayed thumb is not moved.

The thumb also does not move if you use the arrowheads, or if you manually set the thumb for either of the scroll bars from the APL session:

      )cs demo
'F1.UPDOWN' eWS 'Thumb' 200
'F1.LEFTRIGHT' eWS 'Thumb' 200