core: Add a `root_clip` method for `Stage` and make everything grabbing depth 0 off the stage use it

core/src/avm1/activation.rs

@@ -343,7 +343,7 @@ impl<'a, 'gc, 'gc_context> Activation<'a, 'gc, 'gc_context> {
     ) -> Self {
         let version = context.swf.version();
         let globals = context.avm1.global_object_cell();
-        let level0 = context.stage.child_by_depth(0).unwrap();
+        let level0 = context.stage.root_clip();
 
         Self::from_nothing(context, id, version, globals, level0)
     }

core/src/avm1/globals/sound.rs

@@ -8,7 +8,7 @@ use crate::avm1::property::Attribute;
 use crate::avm1::{Object, ScriptObject, SoundObject, TObject, Value};
 use crate::avm_warn;
 use crate::character::Character;
-use crate::display_object::{SoundTransform, TDisplayObject, TDisplayObjectContainer};
+use crate::display_object::{SoundTransform, TDisplayObject};
 use gc_arena::MutationContext;
 
 /// Implements `Sound`
@@ -204,8 +204,8 @@ fn attach_sound<'gc>(
         let name = name.coerce_to_string(activation)?;
         let movie = sound_object
             .owner()
-            .or_else(|| activation.context.stage.child_by_depth(0))
+            .unwrap_or_else(|| activation.context.stage.root_clip())
-            .and_then(|o| o.movie());
+            .movie();
         if let Some(movie) = movie {
             if let Some(Character::Sound(sound)) = activation
                 .context
@@ -539,8 +539,8 @@ fn stop<'gc>(
             let name = name.coerce_to_string(activation)?;
             let movie = sound
                 .owner()
-                .or_else(|| activation.context.stage.child_by_depth(0))
+                .unwrap_or_else(|| activation.context.stage.root_clip())
-                .and_then(|o| o.movie());
+                .movie();
             if let Some(movie) = movie {
                 if let Some(Character::Sound(sound)) = activation
                     .context

core/src/avm1/timer.rs

@@ -9,7 +9,6 @@
 use crate::avm1::object::search_prototype;
 use crate::avm1::{Activation, ActivationIdentifier, Object, TObject, Value};
 use crate::context::UpdateContext;
-use crate::display_object::TDisplayObjectContainer;
 use gc_arena::Collect;
 use std::collections::{binary_heap::PeekMut, BinaryHeap};
 
@@ -40,7 +39,7 @@ impl<'gc> Timers<'gc> {
 
         let version = context.swf.header().version;
         let globals = context.avm1.global_object_cell();
-        let level0 = context.stage.child_by_depth(0).expect("root movie");
+        let level0 = context.stage.root_clip();
 
         let mut activation = Activation::from_nothing(
             context.reborrow(),

core/src/context.rs

@@ -160,7 +160,7 @@ impl<'a, 'gc, 'gc_context> UpdateContext<'a, 'gc, 'gc_context> {
             self.audio,
             self.gc_context,
             self.action_queue,
-            self.stage.child_by_depth(0).expect("root movie"),
+            self.stage.root_clip(),
         );
     }
 

core/src/display_object/stage.rs

@@ -236,6 +236,14 @@ impl<'gc> Stage<'gc> {
             );
         }
     }
+
+    /// Obtain the root movie on the stage.
+    ///
+    /// `Stage` guarantees that there is always a movie clip at depth 0.
+    pub fn root_clip(self) -> DisplayObject<'gc> {
+        self.child_by_depth(0)
+            .expect("Stage must always have a root movie")
+    }
 }
 
 impl<'gc> TDisplayObject<'gc> for Stage<'gc> {

core/src/external.rs

@@ -7,7 +7,6 @@ use crate::avm1::{
     AvmString as Avm1String, Object as Avm1Object, ScriptObject as Avm1ScriptObject,
 };
 use crate::context::UpdateContext;
-use crate::display_object::TDisplayObjectContainer;
 use gc_arena::Collect;
 use std::collections::BTreeMap;
 
@@ -204,7 +203,7 @@ impl<'gc> Callback<'gc> {
     ) -> Value {
         match self {
             Callback::Avm1 { this, method } => {
-                let base_clip = context.stage.child_by_depth(0).unwrap();
+                let base_clip = context.stage.root_clip();
                 let swf_version = context.swf.version();
                 let globals = context.avm1.global_object_cell();
                 let mut activation = Avm1Activation::from_nothing(

core/src/focus_tracker.rs

@@ -43,7 +43,7 @@ impl<'gc> FocusTracker<'gc> {
 
         log::info!("Focus is now on {:?}", focused_element);
 
-        let level0 = context.stage.child_by_depth(0).unwrap();
+        let level0 = context.stage.root_clip();
         Avm1::notify_system_listeners(
             level0,
             context.swf.version(),

core/src/loader.rs

@@ -5,7 +5,7 @@ use crate::avm1::{Avm1, AvmString, Object, TObject, Value};
 use crate::avm2::Domain as Avm2Domain;
 use crate::backend::navigator::OwnedFuture;
 use crate::context::{ActionQueue, ActionType};
-use crate::display_object::{DisplayObject, MorphShape, TDisplayObject, TDisplayObjectContainer};
+use crate::display_object::{DisplayObject, MorphShape, TDisplayObject};
 use crate::player::{Player, NEWEST_PLAYER_VERSION};
 use crate::property_map::PropertyMap;
 use crate::tag_utils::SwfMovie;
@@ -454,9 +454,7 @@ impl<'gc> Loader<'gc> {
                         _ => unreachable!(),
                     };
 
-                    if let Some(root) = uc.stage.child_by_depth(0) {
+                    replacing_root_movie = DisplayObject::ptr_eq(clip, uc.stage.root_clip());
-                        replacing_root_movie = DisplayObject::ptr_eq(clip, root);
-                    }
 
                     clip.as_movie_clip().unwrap().unload(uc);
 

core/src/player.rs

@@ -777,7 +777,7 @@ impl Player {
             // Fire event listener on appropriate object
             if let Some((listener_type, event_name, args)) = listener {
                 context.action_queue.queue_actions(
-                    context.stage.child_by_depth(0).expect("root level"),
+                    context.stage.root_clip(),
                     ActionType::NotifyListeners {
                         listener: listener_type,
                         method: event_name,
@@ -917,7 +917,7 @@ impl Player {
         let mut is_action_script_3 = false;
         self.mutate_with_update_context(|context| {
             let mut morph_shapes = fnv::FnvHashMap::default();
-            let root = context.stage.child_by_depth(0).expect("root level");
+            let root = context.stage.root_clip();
             root.as_movie_clip()
                 .unwrap()
                 .preload(context, &mut morph_shapes);
@@ -1249,8 +1249,8 @@ impl Player {
 
             *current_frame = update_context
                 .stage
-                .child_by_depth(0)
+                .root_clip()
-                .and_then(|root| root.as_movie_clip())
+                .as_movie_clip()
                 .map(|clip| clip.current_frame());
 
             // Hovered object may have been updated; copy it back to the GC root.