[css-overflow-4] Make scrollbar-gutter:stable apply to overflow:hidden

See w3c#4674

Author: frivoal

css-overflow-4/Overview.bs

@@ -271,7 +271,7 @@ Whether scrollbars appear on the start or end edge of the box is UA defined.
 For <a>classic scrollbars</a>,
 the width of the <a>scrollbar gutter</a> is the same as the width of the scrollbar.
 For <a>overlay scrollbars</a>,
-the width of the <a>scrollbar gutter</a> is UA defined.
+the width of the <a>scrollbar gutter</a>, if present, is UA defined.
 However, it must not be 0,
 and it must not change based on user interactions with the page or the scrollbar
 even if the scrollbar itself changes.
@@ -288,10 +288,11 @@ The values of this property have the following meaning:
 
 	<dt><dfn>stable</dfn>
 	<dd>The <a>scrollbar gutter</a> is present when
-	'overflow' is ''overflow/scroll'' or ''overflow/auto''
-	and the scrollbar is a <a>classic scrollbar</a>
-	even if the box is not overflowing,
-	but not when the scrollbar is an <a>overlay scrollbar</a>.
+	the scrollbar is a <a>classic scrollbar</a>
+	when 'overflow' is ''overflow/hidden', ''overflow/scroll'', or ''overflow/auto'',
+	regardless of whether the box is actually overflowing.
+	The <a>scrollbar gutter</a> is not present
+	when the scrollbar is an <a>overlay scrollbar</a>.
 
 	<dt><dfn>always</dfn>
 	<dd>The <a>scrollbar gutter</a> is always present when
@@ -377,7 +378,26 @@ showing in which case space is reserved for the <a>scrollbar gutter</a>.
 		<td>yes
 		<td>yes
 	<tr>
-		<th rowspan=3>''overflow/visible'', ''overflow/hidden'', ''overflow/clip''
+		<th rowspan=3>''overflow/hidden''
+		<th>''scrollbar-gutter/auto''
+		<td>
+		<td>
+		<td>
+		<td>
+	<tr>
+		<th>''scrollbar-gutter/stable''
+		<td>yes
+		<td>yes
+		<td>
+		<td>
+	<tr>
+		<th>''scrollbar-gutter/always''
+		<td>if ''force''
+		<td>if ''force''
+		<td>if ''force''
+		<td>if ''force''
+	<tr>
+		<th rowspan=3>''overflow/visible'', ''overflow/clip''
 		<th>''scrollbar-gutter/auto''
 		<td>
 		<td>