[css-break-3] Define box-split fragments to be affected by box-decoration-break. Fixes #1706

Author: fantasai

css-break-3/Overview.bs

@@ -1040,10 +1040,19 @@ Fragmented Borders and Backgrounds: the 'box-decoration-break' property</h3>
     on the left, the value ''slice'', on the right the value ''clone''.
   </div>
 
-  <p>UAs may also apply 'box-decoration-break' to control rendering
-  at bidi-imposed breaks, i.e. when bidi reordering causes an inline
-  to split into non-contiguous fragments. Otherwise such breaks are
-  always handled as ''slice''.
+  <p>UAs should also apply 'box-decoration-break'
+  to control rendering at bidi-imposed breaks--
+  i.e. when bidi reordering causes an inline to split into non-contiguous fragments--
+  and/or at display-type&ndash;imposed breaks--
+  i.e. when a higher-level <a>display type</a>
+  (such as a <a>block-level box</a> / <a href="https://www.w3.org/TR/css-multicol-1/#spanning-columns">column spanner</a>)
+  splits an incompatible ancestor
+  (such as an <a>inline box</a> / <a>block container</a>).
+  Otherwise such breaks must be handled as ''slice''.
+  See [[css-writing-modes-3#bidi-algo|Applying the Bidirectional Reorderign Algorithm]]
+  in [[css-writing-modes-3#text-direction|CSS Writing Modes]],
+  <a href="https://www.w3.org/TR/CSS2/visuren.html#box-gen">CSS2&sect;9.2 Block-level elements and block boxes</a>,
+  and <a href="https://www.w3.org/TR/css-multicol-1/#spanning-columns">CSS Multi-column Layout &sect;6 Spanning Columns</a>.
 
   <p class=note>
   For inline elements, which side of a fragment is considered the broken edge
@@ -1162,10 +1171,46 @@ Changes</h2>
     <a href="https://www.w3.org/TR/2017/CR-css-break-3-20170209/">9 February 2017 Candidate Recommendation</a>:
 
   <ul>
-    <li id="change-2018-aliasing">Clarify the mechanism of aliasing for the page-break-* properties
-    <blockquote>
-      <p>by treating the 'page-break-*' properties as <del>shorthands</del> <ins>[=legacy shorthands=]</ins> for the 'break-*' properties
-    </blockquote>
+    <li id="change-2018-bidi-split-breaks">
+      Clarify that bidi-imposed breaks and block-in-inline breaks create fragments,
+      and that their formatting should be controlled by 'box-decoration-break'.
+      (<a href="https://github.com/w3c/csswg-drafts/issues/1706">Issue 1706</a>)
+      <blockquote>
+        <ins><p class="note">
+        A box can be broken into multiple <a>fragments</a>
+        also due to bidi reordering of text
+        (see [[css-writing-modes-3#bidi-algo|Applying the Bidirectional Reorderign Algorithm]]
+        in [[css-writing-modes-3#text-direction|CSS Writing Modes]])
+        or higher-level <a>display type</a> box splitting,
+        e.g. <a href="https://www.w3.org/TR/CSS2/visuren.html#img-anon-block">block-in-inline splitting</a>
+        (see <a href="https://www.w3.org/TR/CSS2/visuren.html#box-gen">CSS2&sect;9.2</a>)
+        or <a href="https://www.w3.org/TR/css-multicol-1/#spanning-columns">column-spanner-in-block</a> splitting
+        (see <a href="https://www.w3.org/TR/css-multicol-1/#spanning-columns">CSS Multi-column Layout</a>).
+        The division into <a>box fragments</a> in these cases
+        does not depend on layout (sizing/positioning of content).</p></ins>
+      </blockquote>
+      <blockquote>
+        <p>UAs <del>may</del><ins>should</ins> also apply 'box-decoration-break' to control rendering
+        at bidi-imposed breaks--
+        i.e. when bidi reordering causes an inline
+        to split into non-contiguous fragments<ins>--
+        and/or at display-type&ndash;imposed breaks--
+        i.e. when a higher-level <a>display type</a>
+        (such as a <a>block-level box</a> / <a href="https://www.w3.org/TR/css-multicol-1/#spanning-columns">column spanner</a>)
+        splits an incompatible ancestor
+        (such as an <a>inline box</a> / <a>block container</a>)</ins>.
+        Otherwise such breaks must be handled as ''slice''.
+        <ins>See [[css-writing-modes-3#bidi-algo|Applying the Bidirectional Reorderign Algorithm]]
+        in [[css-writing-modes-3#text-direction|CSS Writing Modes]],
+        <a href="https://www.w3.org/TR/CSS2/visuren.html#box-gen">CSS2&sect;9.2 Block-level elements and block boxes</a>,
+        and <a href="https://www.w3.org/TR/css-multicol-1/#spanning-columns">CSS Multi-column Layout &sect;6 Spanning Columns</a>.</ins></p>
+      </blockquote>
+    <li id="change-2018-aliasing">
+      Clarify the mechanism of aliasing for the page-break-* properties.
+      (<a href="https://github.com/w3c/csswg-drafts/issues/866">Issue 866</a>)
+      <blockquote>
+        <p>by treating the 'page-break-*' properties as <del>shorthands</del> <ins>[=legacy shorthands=]</ins> for the 'break-*' properties
+      </blockquote>
   </ul>
 
   <p>The following significant changes were made since the