[css-nav-1] Modify the condition of candidates for custom spatial navigation container

Close #3965

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -1088,8 +1088,8 @@ run the following steps:
     Note: <var>focusables</var> may be empty
 4. Let <var>insideArea</var> be
     * the <a>optimal viewing region</a> of <var>C</var> if <var>C</var> is a <a>scroll container</a>,
-    * the viewport of C's <a for="/">browsing context</a> if <var>C</var> is a <a>Document</a>,
-    * the <a>border box</a> of <var>C</var> otherwise.
+    * the viewport of <var>C</var>'s <a for="/">browsing context</a> if <var>C</var> is a <a>Document</a>,
+    * the <a>optimal viewing region</a> of <var>C</var>'s nearest ancestor <a>scroll container</a> otherwise.
 5. Let <var>visibles</var> be the subset of items in <var>focusables</var>
     whose <a>boundary box</a>
     is at least partly within <var>insideArea</var>.
@@ -1127,10 +1127,9 @@ run the following steps:
 3. Let <var>insideArea</var> be
     * the <a>optimal viewing region</a> of <var>searchOrigin</var> if <var>searchOrigin</var> is a <a>scroll container</a>,
     * the viewport if <var>searchOrigin</var> is a <a>Document</a>,
-    * the <a>border box</a> of <var>searchOrigin</var> otherwise.
-4. Let <var>insiders</var> be the subset of <var>candidates</var> items
-    whose [=DOM anchor=] are descendants of <var>searchOrigin</var>
-    and whose <a>boundary box</a>'s
+    * the <a>optimal viewing region</a> of <var>searchOrigin</var>’s nearest ancestor <a>scroll container</a> otherwise.
+4. Let <var>insiders</var> be the subset of <var>candidates</var>
+    whose <a>boundary box</a>'s
     * top edge is below the top edge of <var>insideArea</var> if <var>dir</var> is {{SpatialNavigationDirection/down}}
     * bottom edge is above the bottom edge of <var>insideArea</var> if <var>dir</var> is {{SpatialNavigationDirection/up}}
     * right edge is left of the right edge of <var>insideArea</var> if <var>dir</var> is {{SpatialNavigationDirection/left}}