How can I detect scrolling in a field?

[graham samuel]

I'm sure this is blindingly obvious, but not to me.

If I have a scrolling field (say "F1") with n lines in it, and next to 
it on the same card I have a scrolling field of identical size also 
containing n lines, where each line contains information which relates 
to the same line in the other field (say "F2"), what is the easiest way 
to keep the fields in step when the user chooses to scroll one of them? 
In the docs, it says

>  When the user clicks in the scroll bar to scroll a field, the setting 
> of the field's vScroll property changes automatically.

What it doesn't say is if any message is sent when the user uses the 
mouse to scroll. I thought  mouseDown and mouseUp would be sent, but  
if I script

on mouseUp
   set the VScroll of field "F2" to the Vscroll of me
end mouseUp

in field F1, I get precisely nothing, and indeed I can show that 
"mouseUp" isn't sent if I click anywhere in the field, whether in the 
scrollbar or not.

What am I doing wrong?

TIA

Graham

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Graham Samuel / The Living Fossil Co. / UK and France