[css3-box] How do scrollbars affect width calculation?

Instructions: depress the left mouse button inside the latin text and, while holding down the mouse button, drag it up or down.

Expected result: the computed CSS width should indicate 400px at all times, regardless of presence or absence of vertical scrollbar.

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer dapibus orci quis tristique malesuada. Donec consectetur cursus ipsum, in porta ante posuere in. Quisque ipsum lorem, ornare eu luctus eu, vestibulum sed lectus. Maecenas id mattis leo. Nunc luctus faucibus ipsum, vel facilisis neque lobortis ut. Curabitur interdum in diam id feugiat. Pellentesque cursus ornare posuere. Quisque nec ultricies libero. Sed tincidunt blandit auctor. Mauris vulputate, ante in mattis lacinia, lorem nibh ullamcorper sem, rutrum sodales ipsum dui a lacus. Sed eu risus velit.

computed CSS height:

computed CSS width: : should be 400px at all times.

Actual results: Firefox 52.9.0 ESR, Firefox 26, Epiphany/3.22.7 (Safari/605.1; WebKit 2.18.6) and Opera 12.16 pass this test. Opera 56.0.3037.0, IE 11.0.2, Chrome 32.0.1700.107 and Konqueror 4.12.1 fail this test.