Specify ink overflow extent for box-shadow

Issue w3c#8649

Author: szager-chromium

css-backgrounds-3/Overview.bs

@@ -31,6 +31,7 @@ spec:css2; type:value; text:visible
 spec:css-break-4; type:dfn; text:fragment
 spec:css-color-4; type:property; text:color
 spec:css-color-4; type:value; text:transparent
+spec:css-overflow-3; type:dfn; text:ink overflow area 
 spec:selectors-3; type:selector; text: ::first-letter
 spec:selectors-3; type:selector; text: ::first-line
 spec:css-values-3; type:type; text:<position>
@@ -2095,7 +2096,8 @@ they apply to the top, right, bottom, and left, respectively.
 
 <p>Portions of the border-image that are rendered outside the border
 box do not trigger scrolling. Also such portions are invisible to mouse
-events and do not capture such events on behalf of the element.</p>
+events and do not capture such events on behalf of the element, but
+they are included in the [=ink overflow area=] of the element.</p>
 
 <p class=note>Note that, even though they never cause a scrolling
 mechanism, outset images may still be clipped by an ancestor or by the
@@ -2352,6 +2354,14 @@ Animation type: by computed value,
     </p>
   </div>
 
+  <p>If the ''box-shadow/inset'' keyword is not present,
+  the [=ink overflow area=] of a box shadow is defined by the
+  extent of the [=box-shadow/horizontal offset=], [=box-shadow/vertical offset=],
+  and [=box-shadow/spread distance=] values beyond the element's border-box,
+  plus the ink overflow contributed by the [=box-shadow/blur radius=]
+  as described for the
+  <a class="css" data-link-type="maybe" href="https://drafts.fxtf.org/filter-effects-1/#funcdef-filter-blur" id="ref-for-funcdef-filter-blur">blur()</a> filter function.</p>
+
 <h4 id="shadow-shape">
 Shadow Shape, Spread, and Knockout</h4>