[css-ui-4] Record issue about what fade applies to

Author: frivoal

css-ui-4/Overview.bs

@@ -113,6 +113,13 @@ This specification introduces a new value to the 'text-overflow' property:
     ''1em'' is suggested as a reasonable value.
 </dl>
 
+Issue: How should we deal with
+things overflowing out of the line box,
+or overlapping onto it?
+Should fade apply to the logical content of the line,
+or to the physical area of the line box,
+or the intersection of both?
+
 <div class="example">
 
 	<style>