title: How to calculate the width of the scroll bar?
body: <p>Given a <code>&lt;textarea&gt;</code> with a fixed <code>width</code>, I would like its "active width" to be constant (in <code>px</code>). By "active width" I mean the area where the text appears. </p>
  
  <p>When the vertical scroll bar doesn't appear, the "active width" equals to <code>width</code>. But, when the vertical scroll bar appears, the "active width" becomes smaller than <code>width</code> (I guess smaller exactly by the width of the scroll bar).</p>
  
  <p>I thought to identify whether the vertical scroll bar appears or not, and if yes, to increase the width of the <code>&lt;textarea&gt;</code> by the width of the scroll bar. How could I identify the width of the scroll bar?</p>
  
  <p>Is there a better approach?</p>
  
  <p>(I'm interested in Firefox, if it makes the life easier.)</p>