[css-ui-3] Use the hotspot and hit testing to pick which element determines the cursor

Author: frivoal

css-ui/Overview.bs

@@ -973,10 +973,22 @@ then only the end ellipsis/string should be rendered.
 </tbody></table>
 
 This property specifies the type of cursor to be displayed for the pointing device
-when within the element's <a>border edge</a>.
+when the cursor's hotspot is within the element's <a>border edge</a>.
 
 Note: As per [[CSS3-BACKGROUND]], the <a>border edge</a> is affected by 'border-radius'.
 
+In the case of overlapping elements,
+which element determines the type of cursor
+is based on hit testing:
+the element determining the cursor
+is the one that would receive a click
+initiated from this position.
+
+Note: The specifics of hit testing
+are out of scope of this specification.
+Hit testing will hopefully be defined
+in a future revision of CSS or HTML.
+
 User agents may ignore the cursor property over native user-agent controls such as scrollbars, resizers, or other native UI widgets e.g. those that may be used inside some user agent specific implementations of form elements.
 
 Values have the following meanings: