[css-backgrounds-3] Only apply 0 < 1 radius-to-spread ratio adjustment for outset shadows; inset shadows don't need it because their radii shrink rather than grow. #1900

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -2932,11 +2932,13 @@ Shadow Shape, Spread, and Knockout</h4>
 
   <p>To preserve the box's shape when spread is applied,
   the corner radii of the shadow are also increased (decreased, for inner shadows)
-  from the border-box radii by adding the <i>spread distance</i> (and flooring at zero).
+  from the border-box radii by adding (subtracting)
+  the <i>spread distance</i> (and flooring at zero).
   However, in order to create a sharper corner when the border radius is small
   (and thus ensure continuity between round and sharp corners),
   when the <i>border radius</i> is less than the <i>spread distance</i>,
-  the <i>spread distance</i> is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
+  the <i>spread distance</i> of an outer shadow
+  is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
   where <var>r</var> is the ratio of the border radius to the <i>spread distance</i>,
   in calculating the corner radii of the spread shadow shape.
   For example, if the border radius is 10px and the <i>spread distance</i> is 20px (<var>r</var> = .5),
@@ -3062,11 +3064,29 @@ Changes</h2>
 Changes since the 17 October 2017 Candidate Recommendation</h3>
 
 <ul>
-  <li>Inverted order of <<shadow>> grammar to match browser serialization and 'text-shadow'/''drop-shadow()''.
+  <li>
+    Inverted order of <<shadow>> grammar to match browser serialization and 'text-shadow'/''drop-shadow()''.
       (<a href="https://github.com/w3c/csswg-drafts/issues/2305">Issue 2305</a>)
       <blockquote>
         <pre><<shadow>> = <del>inset</del><ins><<color>></ins>? &amp;&amp; <<length>>{2,4} &amp;&amp; <del><<color>></del><ins>inset</ins>?</pre>
       </blockquote>
+  <li>
+    Spread radius adjustment is only applied to outer shadows (and positive margins)--
+    i.e. cases where the radius of curvature grows, rather than shrinks.
+    (<a href="https://github.com/w3c/csswg-drafts/issues/1900">Issue 1900</a>)
+      <blockquote>
+        <p>To preserve the box's shape when spread is applied,
+        the corner radii of the shadow are also increased (decreased, for inner shadows)
+        from the border-box radii by adding (subtracting)
+        the <i>spread distance</i> (and flooring at zero).
+        However, in order to create a sharper corner when the border radius is small
+        <ins>(and thus ensure continuity between round and sharp corners)</ins>,
+        when the <i>border radius</i> is less than the <i>spread distance</i>,
+        the <i>spread distance</i> <ins>of an outer shadow</ins>
+        is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
+        where <var>r</var> is the ratio of the border radius to the <i>spread distance</i>,
+        in calculating the corner radii of the spread shadow shape.
+      </blockquote>
 </ul>
 
 <h3 id="changes-2014-09">