avm1: Support MovieClip.tabChildren

The property `MovieClip.tabChildren` allows changing the behavior of
tab ordering hierarchically. When set to `false`, it excludes the whole
subtree represented by the movie clip from tab ordering.

core/src/avm1/globals/movie_clip.rs

@@ -119,6 +119,7 @@ const PROTO_DECLS: &[Declaration] = declare_properties! {
     // NOTE: `tabEnabled` is not a built-in property of MovieClip.
     // NOTE: `tabIndex` is not enumerable in MovieClip, contrary to Button and TextField
     "tabIndex" => property(mc_getter!(tab_index), mc_setter!(set_tab_index); DONT_ENUM | VERSION_6);
+    // NOTE: `tabChildren` is not a built-in property of MovieClip.
 };
 
 /// Implements `MovieClip`

core/src/display_object/container.rs

@@ -475,12 +475,19 @@ pub trait TDisplayObjectContainer<'gc>:
         RenderIter::from_container(self.into())
     }
 
+    fn is_tab_children(&self, _context: &mut UpdateContext<'_, 'gc>) -> bool {
+        true
+    }
+
     fn fill_tab_order(
         &self,
         tab_order: &mut Vec<DisplayObject<'gc>>,
         context: &mut UpdateContext<'_, 'gc>,
     ) {
-        // TODO Add support for `tabChildren`
+        if !self.is_tab_children(context) {
+            return;
+        }
+
         for child in self.iter_render_list() {
             if child.is_tab_enabled(context) {
                 tab_order.push(child);

core/src/display_object/movie_clip.rs

@@ -3051,6 +3051,18 @@ impl<'gc> TDisplayObjectContainer<'gc> for MovieClip<'gc> {
     fn raw_container_mut(&self, gc_context: &Mutation<'gc>) -> RefMut<'_, ChildContainer<'gc>> {
         RefMut::map(self.0.write(gc_context), |this| &mut this.container)
     }
+
+    /// The property `MovieClip.tabChildren` allows changing the behavior of
+    /// tab ordering hierarchically.
+    /// When set to `false`, it excludes the whole subtree represented by
+    /// the movie clip from tab ordering.
+    ///
+    /// _NOTE:_
+    /// According to the AS2 documentation, it should affect only automatic tab ordering.
+    /// However, that does not seem to be the case, as it also affects custom ordering.
+    fn is_tab_children(&self, context: &mut UpdateContext<'_, 'gc>) -> bool {
+        self.get_avm1_boolean_property(context, "tabChildren", true)
+    }
 }
 
 impl<'gc> TInteractiveObject<'gc> for MovieClip<'gc> {

core/src/focus_tracker.rs

@@ -108,6 +108,8 @@ impl<'gc> FocusTracker<'gc> {
             first
         };
 
-        self.set(next.copied(), context);
+        if next.is_some() {
+            self.set(next.copied(), context);
+        }
     }
 }