[css-nav-1] Modify how to select candidates for partially overlapped case

Modify the way to select candidates for partially overlapped element with the search origin element.

More detail condition about how elements are overlapped leaves to the UA.

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -391,7 +391,6 @@ the following requirements apply:
         as responses to pressing arrow keys would also be possibilities.
     </div>
 
-
 <div class=api>
 <h2 id="js-api">
 JavaScript API</h2>
@@ -1227,8 +1226,15 @@ run the following steps:
                 * bottom edge is above the top edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/up}}
                 * right edge is left of the left edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/left}}
                 * left edge is right of the right edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/right}}
-            * the item partially overlaps with <var>searchOrigin</var> and
-                its two edges which are orthogonal to <var>dir</var> should be on the navigation direction of the respective ones of <var>searchOrigin</var>.
+            * the item partially overlaps with <var>searchOrigin</var> and its <a>boundary box</a>'s
+                * top edge is below the top edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/down}}
+                * bottom edge is above the bottom edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/up}}
+                * right edge is left of the right edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/left}}
+                * left edge is right of the left edge of <var>searchOrigin</var>'s <a>boundary box</a> if <var>dir</var> is {{SpatialNavigationDirection/right}}
+                
+                NOTE: More detail condition about how the element is overlapped with the search origin affects the sequence of focus movement.
+                The sequence of focus movement is related to UX, so it depends on the UA-defined mechanism.
+
         2. For each <var ignore="">candidate</var> in <var>candidates</var>,
             [=find the shortest distance=] between <var>searchOrigin</var>.
         3. Return the item of the <var>candidates</var> set that has the smallest distance.