Peter

I believe to style that line you’ll need to create your own custom skin and draw/modify the line using the drawing API.
The default ComboBox skin class is \frameworks\projects\framework\src\mx\skins\halo\ComboBoxArrowSkin.as, and the line is drawn in a few places (but seemingly always in an if (!arrowOnly){...} block):

if (!arrowOnly)
{
    // line
    drawRoundRect(
        w - 22, 4, 1, h - 8, 0,
        borderColor, 1);
    drawRoundRect(
        w - 21, 4, 1, h - 8, 0,
        0xFFFFFF, 0.2);
}

So in this example above (taken from the default skin file), there are two lines that get drawn. The first drawRoundRect() method call draws a vertical line with the specified borderColor style. The second drawRoundRect() method call draws a 20% alpha white shadow just to the right of the first line.

If you want to remove the vertical line completely, just delete each of the if (!arrowOnly){...} blocks in each of the case statements. Also, from the looks of it, the borderColor may be getting overridden by the dropdownBorderColor if the ComboBox control is in an error state (for example, the errorString property is not an empty string).

Good question though, I’ll see if I can convert this into a full blog example where I remove that vertical separator line completely.

Peter