[css-values-4] Large, small and dynamic viewport units (lvh, svh & dvh) don't solve top level horizontal scrolling use cases

[lpd-au]

Relevant Spec: https://drafts.csswg.org/css-values-4/#viewport-variants

Imagine an empty page styled with:

:root {
  width: 1000vw;
  height: 100%;
}

body {
  width: 100%;
  height: 100%;
  margin: 0;
  background: linear-gradient(to right, white, red);
}

Currently, on desktop it renders with only a horizontal scrollbar, like so:

But of course on mobile with retractable chrome, this is problematic:

Ideally, this could be solved using a solution such as:

:root {
  height: 100lvh;
}

This would work on mobile; however, the spec currently states that:

In all cases, scrollbars are assumed not to exist. Note however that the initial containing block's size is affected by the presence of scrollbars on the viewport.

So on desktop, you'd get the frustrating effect of a horizontal scrollbar, like so:

When used throughout the document, I understand the scrollbars limitation is for a good reason: #6026 (comment). Irrespective of whether this general limitation can be overcome in level 4, surely the root element is a special case where a reasonable solution can be found? If not investigated now while a large number of new units are being specified, would it become too complex later?

[tabatkins]

Closing as a dupe of #6026, which has most of the discussion.