core: Assign `rootN` format names to top-level AVM2 movie clips.

core/src/avm1/activation.rs

@@ -2776,6 +2776,7 @@ impl<'a, 'gc, 'gc_context> Activation<'a, 'gc, 'gc_context> {
             .into();
 
             level.set_depth(self.context.gc_context, level_id as i32);
+            level.set_default_root_name(&mut self.context);
             self.context.levels.insert(level_id, level);
             level.post_instantiation(&mut self.context, level, None, Instantiator::Movie, false);
 

core/src/display_object.rs

@@ -949,6 +949,18 @@ pub trait TDisplayObject<'gc>:
         }
     }
 
+    /// Assigns a default root name to this object.
+    ///
+    /// The default root names change based on the AVM configuration of the
+    /// clip; AVM2 clips get `rootN` while AVM1 clips get blank strings.
+    fn set_default_root_name(&self, context: &mut UpdateContext<'_, 'gc, '_>) {
+        if !matches!(self.object2(), Avm2Value::Undefined) {
+            self.set_name(context.gc_context, &format!("root{}", self.depth() + 1));
+        } else if !matches!(self.object(), Avm1Value::Undefined) {
+            self.set_name(context.gc_context, "");
+        }
+    }
+
     fn bind_text_field_variables(&self, activation: &mut Activation<'_, 'gc, '_>) {
         // Check all unbound text fields to see if they apply to this object.
         // TODO: Replace with `Vec::drain_filter` when stable.

core/src/player.rs

@@ -377,7 +377,7 @@ impl Player {
                 None
             };
             root.post_instantiation(context, root, flashvars, Instantiator::Movie, false);
-            root.set_name(context.gc_context, "");
+            root.set_default_root_name(context);
             context.levels.insert(0, root);
 
             // Load and parse the device font.