[css-values-4] Make unconditional scrollbars on the root finally affect viewport unit sizes. #6026

Author: tabatkins

css-values-4/Overview.bs

@@ -2234,14 +2234,21 @@ The Large, Small, and Dynamic Viewport Sizes</h5>
 		and therefore have no effect on any of the [=viewport-percentage lengths=].
 		(Typically on-screen keyboards will fit into this category.)
 
-	In all cases, scrollbars are assumed not to exist.
-	<span class='note'>Note however that the [=initial containing block=]'s size
-	<em>is</em> affected by the presence of scrollbars on the viewport.</span>
-
-	ISSUE: <a href="https://www.w3.org/TR/css-values-3/#viewport-relative-lengths">Level 3 assumes scrollbars never exist</a> because it was hard to implement
-	and only Firefox bothered to do so.
-	This is <a href="https://github.com/w3c/csswg-drafts/issues/1766#issuecomment-460470368">making authors unhappy</a>.
-	Can we improve here?
+	In all cases,
+	if the value of 'overflow' or 'scrollbar-gutter' on the [=root element=] in either axis
+	would cause scrollbars to appear (or space to be reserved for them) unconditionally
+	(for example, ''overflow: scroll'', but not ''overflow: auto''),
+	the [=computed values=] of the [=viewport-percentage lengths=] in that axis
+	are reduced in accordance with the [=initial containing block=].
+	Otherwise,
+	and always in the case of [=media queries=],
+	the [=viewport-percentage lengths=] are sized
+	assuming that scrollbars do not exist
+	(even if this diverges from the [=initial containing block=]).
+
+	Note: The value of 'overflow' on [=the body element=]
+	can sometimes affect the presence of scrollbars on the [=root element=].
+	This <em>does not</em> affect the size of viewport units, however.
 
 <h5 id="viewport-relative-units" class="no-toc">
 The Various Viewport-relative Units</h5>
@@ -5520,6 +5527,10 @@ Recent Changes</h3>
 		(<a href="https://github.com/w3c/csswg-drafts/issues/9713">Issue 9713</a>)
 	* Generally fixed how type inference handles percentages.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/10017">Issue 10017</a>)
+	* Restored dependency of [=viewport-percentage lengths=] on ''overflow: scroll''
+		and added one on 'scrollbar-gutter'
+		to make it possible for 100 of these units to actually match the [=initial containing block=].
+		(<a href="https://github.com/w3c/csswg-drafts/issues/6026">Issue 6026</a>)
 
 	Substantial changes since <a href="https://www.w3.org/TR/2023/WD-css-values-4-20231027/">27 October 2023 WD</a>: