The 'whitebox' appears when the container's height/width are such that both horizontal and vertical scrollbar appear. This tiny 'whitebox' appears on the lower-right corner at the intersection of two scrollbars. Our application's theme is black and hence this white box was totally unacceptable. Following two posts by Tommy helped my fix the problem:
1) http://www.tommyb.com/2006/11/30/flex-using-custom-scrollbar-skins-youll-run-into-this-problem/
2) http://www.tommyb.com/2007/04/29/revisiting-the-flex-whitebox-issue/
The two solutions, when combined, works for me. :)

1) http://www.tommyb.com/2006/11/30/flex-using-custom-scrollbar-skins-youll-run-into-this-problem/
2) http://www.tommyb.com/2007/04/29/revisiting-the-flex-whitebox-issue/
The two solutions, when combined, works for me. :)
