[css-backgrounds-3] Handle negative spread radius of inner shadows. #1900 (comment)

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -1995,6 +1995,8 @@ the margin is multiplied by the proportion 1 + (r-1)3,
 where r is the ratio of the border radius to the margin,
 in calculating the corner radii of the margin box shape.
 
+
+
 Corner Clipping
 
 Although border images are not affected
@@ -2921,8 +2923,10 @@ Shadow Shape, Spread, and Knockout
   the spread distance (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 border radius is less than the spread distance,
-  the spread distance of an outer shadow
+  when the border radius is less than the spread distance
+  (or in the case of an inner shadow,
+  less than the absolute value of a negative spread distance),
+  the spread distance
   is multiplied by the proportion 1 + (r-1)3,
   where r is the ratio of the border radius to the spread distance,
   in calculating the corner radii of the spread shadow shape.
@@ -3057,8 +3061,8 @@ Changes since the 17 October 2017 Candidate Recommendation
         <> = inset<>? && <>{2,4} && <>inset?
       
   
-    Spread radius adjustment is only applied to outer shadows (and positive margins)--
-    i.e. cases where the radius of curvature grows, rather than shrinks.
+    Spread radius adjustment is only applied to shadows and margins
+    where the radius of curvature grows, rather than shrinks.
     (Issue 1900)
       
         To preserve the box's shape when spread is applied,
@@ -3067,8 +3071,10 @@ Changes since the 17 October 2017 Candidate Recommendation
         the spread distance (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 border radius is less than the spread distance,
-        the spread distance of an outer shadow
+        when the border radius is less than the spread distance
+        (or in the case of an inner shadow,
+        less than the absolute value of a negative spread distance),
+        the spread distance
         is multiplied by the proportion 1 + (r-1)3,
         where r is the ratio of the border radius to the spread distance,
         in calculating the corner radii of the spread shadow shape.