This seems like a bit of a strange decision by the spec authors.
Take a look at the following codepen:
Why does 100vw include the scrollbar width? The result of this is that if there’s a vertical scrollbar on the page, setting an element to 100vw causes a horizontal scrollbar. This seems like a very poor implementation to me, surely 100vw should be the visible area (ie excluding the scrollbar)
On pages that don’t have a vertical scrollbar, no horizontal scrollbar is created:
Where this gets particualrly annoying is when you’re trying to add several elements up to get to 100vw, you have to shoot for ~98 to be safe and even then it’s not perfect