[css-writing-modes] Use nearest available scrollport in favor of ICB. Fixes #1391.

Author: fantasai

css-writing-modes-3/Overview.bs

@@ -2015,8 +2015,10 @@ Available Space in Orthogonal Flows</h4>
     but its <i>available inline space</i> to be indefinite.
     In such cases a percentage of the containing block’s <i>inline size</i> cannot be defined,
     and <i>inline axis</i> computations cannot be resolved.
-    In these cases, the initial containing block's size is used
-    as a <i>fallback</i> variable in place of the <i>available inline space</i>
+    In these cases, the nearest ancestor <a>scrollport</a>’s size
+    (or if there are none, the initial containing block’s size)
+    is used as a <i>fallback</i>
+    in place of the <i>available inline space</i>
     for calculations that require a definite <i>available inline space</i>.
 
   <p>See [[!CSS3-SIZING]] for further details on CSS sizing terminology and concepts.
@@ -2468,6 +2470,9 @@ Privacy and Security Considerations {#priv-sec}
     <li>Deferred the ''sideways-lr'' and ''sideways-rl'' values of 'writing-mode' to Level 4.
     <li>Deferred the ''digits'' value of 'text-combine-upright' to Level 4.
     <li>Deferred the automatic multi-column behavior of orthogonal flows to Level 4.
+    <li>Changed the fallback “available space” for orthogonal flows
+        to use the nearest <a>scrollport</a> where available.
+        (<a href="https://github.com/w3c/csswg-drafts/issues/1391">Issue 1391</a>)
     <li>Added Privacy and Security Considerations section.
   </ul>
 

css-writing-modes/Overview.bs

@@ -2044,8 +2044,10 @@ Available Space in Orthogonal Flows</h4>
     but its <i>available inline space</i> to be indefinite.
     In such cases a percentage of the containing block’s <i>inline size</i> cannot be defined,
     and <i>inline axis</i> computations cannot be resolved.
-    In these cases, the initial containing block's size is used
-    as a <i>fallback</i> variable in place of the <i>available inline space</i>
+    In these cases, the nearest ancestor <a>scrollport</a>’s size
+    (or if there are none, the initial containing block’s size)
+    is used as a <i>fallback</i>
+    in place of the <i>available inline space</i>
     for calculations that require a definite <i>available inline space</i>.
 
   <p>See [[!CSS3-SIZING]] for further details on CSS sizing terminology and concepts.
@@ -2077,7 +2079,8 @@ Auto-sizing Block Containers in Orthogonal Flows</h4>
         that would <a>stretch fit</a> into to the smaller of
         <ul>
           <li>the <i>available space</i>
-          <li>the initial containing block’s <i>size</i>
+          <li>the nearest ancestor <a>scrollport</a>’s <a>size</a>,
+          or, if there is none, the initial containing block’s <i>size</i>
       </dl>
 
       <div class="note">