When user types, it calculates the width/height of the hidden element and copies it to the textarea. Because of this dynamic, you can not style your textarea during expansion and expect it to continue ...

However, the Textarea widget (and underlying HTML element inherently support multiple lines. Even if I adjust the layout to present an initial appearance of a single line (a layout height between ...

I think your best chance is to statically size the textarea AND the related divs using CSS. You don't have to use ems; you could use pixles whichever works better for you situation.