How can I get browser scrollbar width?

I've got few little elements, let say divs (200px x 400px CONSTANT). Inside each of them, there is a paragraph with about 20 lines of text. What I want to do is:

Normally div has overflow: hidden property.
On mouse over (:hover) this property is changed to overflow: auto;, and the scrollbar appears.

What is the problem? I want a little space (padding or margin) between the paragraph text and the scrollbar. Let's say that paragraph has a symmetrical margin: 20px;. But after :hover triggers on, the scrollbar appears, and the whole right side of the paragraph is moved "scrollbar-width" px to the left. Sentences are broken in other lines, and the whole paragraph look different, which is quite annoying and not user friendly. How can I set my CSS, so the only change after hover will be the appearance of scroolbar?

In other words:

/* Normally no scrollbar */
div {display: block; width: 400px; height: 200px; overflow: hidden;}
/* On hover scrollbar appears */
div:hover {overflow: auto;}

/* Paragraph margin predicted for future scrollbar on div */
div p {margin: 20px (20px + scrollbarwidth px) 20px 50px;}
/* With scrollbar margin is symmetrical */
div:hover p {margin: 20px;} /* With scrollbar */

I have done a little snippet for it, with exaplanation of my problem in strong. I hope everything is clear :). I'm searching for a solution for almost two hours now, so I think my question is quite unique and interesting.

div.demo1 {
  width: 400px;
  height: 200px;
  overflow: hidden;
  background: #ddd;
}
div.demo1:hover {
  overflow: auto;
}
div.demo1 p {
  background: #eee;
  margin: 20px;
}
div.demo2 {
  width: 400px;
  height: 200px;
  overflow: hidden;
  background: #ddd;
}
div.demo2:hover {
  overflow: auto;
}
div.demo2 p {
  background: #eee;
  margin: 20px 40px 20px 20px;
}
div.demo2:hover p {
  margin: 20px 20px 20px 20px;
}

<div class="demo1">
  <p>
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
  </p>
</div>
<br>
<br>
<strong>As you can see, on hover right side of the paragraph "moves" left. But I want something like:</strong>
<br>
<br>
<div class="demo2">
  <p>
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
    This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text. This is a long text.
  </p>
</div>

<strong>But with a "perfect" scrollbar width (in this example I used 20px)</strong>

Solution

Scrollbar widths can vary between browsers and operating systems, and unfortunately CSS does not provide a way to detect those widths: we need to use JavaScript.

Other people have solved this problem by measuring the width of the scrollbar on an element:

http://davidwalsh.name/detect-scrollbar-width (original post)
http://jsfiddle.net/a1m6an3u/ (live example)

We create a div .scrollbar-measure, add a scrollbar, and return its size.

// Create the div
var scrollDiv = document.createElement("div");
scrollDiv.className = "scrollbar-measure";
document.body.appendChild(scrollDiv);

// Get the scrollbar width
var scrollbarWidth = scrollDiv.offsetWidth - scrollDiv.clientWidth;
console.warn(scrollbarWidth);

// Delete the div
document.body.removeChild(scrollDiv);

This is fairly straightforward, but it is (obviously) not pure CSS.