Reported by Wolfgang, fixed by Adriàn.

Here is the demonstrator for the issue

Please, try this script:
<verbatim>
::toplevel .t;
set c [::ComboBox .t.c -bwlistbox 1];
::pack $c;
$c configure -values {a b c d e f g h i j k l m n n o p q r s t u};
$c setvalue @20; # Selects item "t";
</verbatim>
Now click on the arrow button.  Note that the list is not scrolled to show the "t".
Click somewhere else (to close the ListBox) and click again on the arrow button.
The "t" item is visible now.
The issue only happens at startup, as we are telling the ListBox to "see" the item before it has ever been displayed.

It is fixed by checkin [36d4ee635f].

THank you,
Harald