// Must set <textarea id="expandOnScroll" style="height: 160px" ...> in HTML
var txtAra = document.getElementById('expandOnScroll');
var ec = 0;
function chkScroll() {  
if ( txtAra.scrollTop > 0 ) { ec++; txtAra.style.height = ( 160+(ec*80+9-ec) )+'px' }
if ( ( (ec < 4) && (window.screen.height>=760) ) || (ec < 3) ) { setTimeout('chkScroll()', 200) }
}
chkScroll();