When you have an edit control associated to a tab with a vertical scrollbar, adding to such an edit control while scrolled up with the tab active will make the control scroll down automatically.
However, if the tab is inactive, it does not scroll down automatically, and there doesn't seem to be a good way to workaround this, I don't think it's intended, if it is we would need a way to force the scrollbar to go down.
Note also that if the scroll is already down and the tab is inactive, it will scroll up and won't keep the scroll down.

Code
dialog tests {
  size -1 -1 200 200
  tab "noedit ", 1, 2 2 198 198
  tab "edit", 2
  edit "", 3, 5 30 190 160, tab 2 read multi vsbar
}
on *:dialog:tests:init:*:var %a 20 | while (%a) { did -a tests 3 line $+ %a $+ $crlf | dec %a }

1) /dialog -om tests tests
2) go to the 'edit' tab, scroll the control up
3) execute "/did -a tests 3 line0 $+ $crlf" and see the scrollbar going down automatically.
4) scroll the control up again, make the first tab the active tab
5) "/did -a tests 3 line-1 $+ $crlf" and this time the control is still scrolled up.
6) you may repeat 4) & 5) again with the scroll being down already this time, to see it moving up.


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