[css-text-3] Hanging characters are scrollable overflow. #2398. Closes PR #2744.

Author: fantasai

css-text-3/Overview.bs

@@ -2133,12 +2133,18 @@ Cursive Scripts</h4>
       does not <a>hang</a> at the end edge of a line.
       At most one punctuation character may <a>hang</a> at each edge of the line.
 
-    <p>A <a lt=hang>hanging</a> punctuation mark is still enclosed inside its inline box
-      and participates in text justification:
-      its character advance width is just not measured when determining
+    <p>A <a lt=hang>hanging</a> punctuation mark
+      is still enclosed inside its parent inline box,
+      is still counted as part of the <a>scrollable overflow region</a> [[!CSS-OVERFLOW-3]],
+      and still participates in text justification:
+      its character advance is just not measured when determining
       how much content fits on the line,
       how much the line's contents need to be expanded or compressed for justification,
       or how to position the content within the line box for text alignment.
+      Effectively, the <a>hanging</a> punctuation mark’s character advance
+      is re-interpreted as an additional negative margin
+      on the affected edge of its parent <a>inline box</a>;
+      the line is otherwise laid out as usual.
 
     <p><dfn lt="stop or comma">Stops and commas</dfn> allowed to <a>hang</a> include:
     <table class="data">