If you’re used to scaling rounded rectangles w/ the scale9Grid, you’re in luck, b/c not much has changed in AS3. scale9Grid is a property of the displayObject class that allows one to scale all but the corners of a displayObject. This way when you enlarge a rounded rectangle, the corners will remain the same size instead of enlarging or warping. Note: Remember to import the rectangle class before using scale9Grid.

Not clear enough? Let’s say that we’re scaling a rectangle from a to b:

If, to achieve the above transformation, we use xscale and yscale, or change the width and height of the rounded rectangle object, we’ll get this:

Look at the rounded corners! Not round, not pretty. So, instead we use scale9Grid, which sets up an invisible grid to determine the focus for scaling. Imagine your rounded rectangle with an invisible grid inside:

The grid makes 9 regions of the original object. The nine regions will act differently, now, when they are scaled:

The top left will not scale,
the top middle will scale horizontally only,
the top right will not scale,
the middle left will scale vertically only,
the middle middle will scale horizontally and vertically,
the middle right will scale vertically only,
the bottom left will not scale,
the bottom middle will scale horizontally only, and
the bottom right will not scale.

This maintains the rounded rectangle look when we scale. In addition, it maintains the stroke size of the rectangle. Of course, you can use the scale9grid for numerous other reasons, but this happens to be the most common in my practice.

For more details, consult the flash language reference. They do a great job explaining it.

This section in particular: Controlling distortion when scaling