Question

Is there a way to reliably tell a browser’s viewport width that includes the scrollbar, but not the rest of browser window)?

None of the properties listed here tell me the width of the screen INCLUDING the scrollbar (if present)

Answer 1

As long as body is 100%, document.body.scrollWidth will work.

Demo: http://jsfiddle.net/ThinkingStiff/5j3bY/

HTML:

<div id="widths"></div>

CSS:

body, html
{
margin: 0;
padding: 0;
width: 100%;
}

div
{
height: 1500px;
}

Script:

var widths = 'viewport width (body.scrollWidth): ' 
    + document.body.scrollWidth + '<br />'
    + 'window.innerWidth: ' + window.innerWidth + '<br />';

document.getElementById( 'widths' ).innerHTML = widths;

I put a tall div in the demo to force a scroll bar.