Expandable text box

I am working to create an expandable text box that will enlarge as the user enters in information. The box starts out as one line, but should end up as multi-line as the form is filled.

The JavaScript is entered as a Keystroke script. It seems to be working for the most part, except when the box increases in size, I must use the resetForm feature to apply the size change. When this happens, the focus is taken away from the text box. I want the user to be able to enter the text without interruption.

It would make sense to use the setFocus feature, although it does not seem to work when I have it in the Keystroke script. I was able to add a button the sets the focus back to the text box, but I want it to be automatic.

Any ideas??

if (event.fieldFull) {
var b = this.getField("Text1");
var aRect = b.rect;
aRect[3] = aRect[3] - 11.5;
b.rect = aRect;

this.resetForm([event.target.name]);
event.change = event.changeEx;
//b.setFocus();
}