Choice popup & scrollbar gets flickering when mouse is pressed & dragged on the scrollbar of the choice.

Step to reproduce:
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1) Run the attached program.
2) Click on the choice. you can see the items of the choice .
3) Click on the scrollbar of the choice & drag the tump of the scrollbar so that it tumb touchs the down arrow.
4) Just drag the mouse over the scrollbar of the choice. If you see scrollbar flicking then bug is reproduced. Even you can see some item of the choice also gets flickering.

This works fine in Tiger RC b63. This problem can be seen in Mustang b42 onward till b77. Hence this is a regression.