Fix transform propagation of orphaned entities (#7264)

# Objective

- Fix #7263

This has nothing to do with #7024. This is for the case where the
user opted to **not** keep the same global transform on update.

## Solution

- Add a `RemovedComponent<Parent>` to `propagate_transforms`
- Add a `RemovedComponent<Parent>` and `Local<Vec<Entity>>` to
`sync_simple_transforms`
- Add test to make sure all of this works.

### Performance note

This should only incur a cost in cases where a parent is removed.

A minimal overhead (one look up in the `removed_components`
sparse set) per root entities without children which transform didn't
change. A `Vec` the size of the largest number of entities removed
with a `Parent` component in a single frame, and a binary search on
a `Vec` per root entities.

It could slow up considerably in situations where a lot of entities are
orphaned consistently during every frame, since
`sync_simple_transforms` is not parallel. But in this situation,
it is likely that the overhead of archetype updates overwhelms
everything.

---

## Changelog

- Fix the `GlobalTransform` not getting updated when `Parent` is removed

## Migration Guide

- If you called `bevy_transform::systems::sync_simple_transforms` and
`bevy_transform::systems::propagate_transforms` (which is not
re-exported by bevy) you need to account for the additional
`RemovedComponents<Parent>` parameter.

---------

Co-authored-by: vyb <vyb@users.noreply.github.com>
Co-authored-by: JoJoJet <21144246+JoJoJet@users.noreply.github.com>

crates/bevy_transform/src/systems.rs

@@ -2,23 +2,39 @@ use crate::components::{GlobalTransform, Transform};
 use bevy_ecs::{
     change_detection::Ref,
     prelude::{Changed, DetectChanges, Entity, Query, With, Without},
+    removal_detection::RemovedComponents,
+    system::{Local, ParamSet},
 };
 use bevy_hierarchy::{Children, Parent};
 
 /// Update [`GlobalTransform`] component of entities that aren't in the hierarchy
 ///
 /// Third party plugins should ensure that this is used in concert with [`propagate_transforms`].
 pub fn sync_simple_transforms(
-    mut query: Query<
-        (&Transform, &mut GlobalTransform),
-        (Changed<Transform>, Without<Parent>, Without<Children>),
-    >,
+    mut query: ParamSet<(
+        Query<
+            (&Transform, &mut GlobalTransform),
+            (Changed<Transform>, Without<Parent>, Without<Children>),
+        >,
+        Query<(Ref<Transform>, &mut GlobalTransform), Without<Children>>,
+    )>,
+    mut orphaned: RemovedComponents<Parent>,
 ) {
+    // Update changed entities.
     query
+        .p0()
         .par_iter_mut()
         .for_each_mut(|(transform, mut global_transform)| {
             *global_transform = GlobalTransform::from(*transform);
         });
+    // Update orphaned entities.
+    let mut query = query.p1();
+    let mut iter = query.iter_many_mut(orphaned.iter());
+    while let Some((transform, mut global_transform)) = iter.fetch_next() {
+        if !transform.is_changed() {
+            *global_transform = GlobalTransform::from(*transform);
+        }
+    }
 }
 
 /// Update [`GlobalTransform`] component of entities based on entity hierarchy and
@@ -30,12 +46,17 @@ pub fn propagate_transforms(
         (Entity, &Children, Ref<Transform>, &mut GlobalTransform),
         Without<Parent>,
     >,
+    mut orphaned: RemovedComponents<Parent>,
     transform_query: Query<(Ref<Transform>, &mut GlobalTransform, Option<&Children>), With<Parent>>,
     parent_query: Query<(Entity, Ref<Parent>)>,
+    mut orphaned_entities: Local<Vec<Entity>>,
 ) {
+    orphaned_entities.clear();
+    orphaned_entities.extend(orphaned.iter());
+    orphaned_entities.sort_unstable();
     root_query.par_iter_mut().for_each_mut(
         |(entity, children, transform, mut global_transform)| {
-            let changed = transform.is_changed();
+            let changed = transform.is_changed() || orphaned_entities.binary_search(&entity).is_ok();
             if changed {
                 *global_transform = GlobalTransform::from(*transform);
             }
@@ -165,6 +186,61 @@ mod test {
     use crate::TransformBundle;
     use bevy_hierarchy::{BuildChildren, BuildWorldChildren, Children, Parent};
 
+    #[test]
+    fn correct_parent_removed() {
+        ComputeTaskPool::init(TaskPool::default);
+        let mut world = World::default();
+        let offset_global_transform =
+            |offset| GlobalTransform::from(Transform::from_xyz(offset, offset, offset));
+        let offset_transform =
+            |offset| TransformBundle::from_transform(Transform::from_xyz(offset, offset, offset));
+
+        let mut schedule = Schedule::new();
+        schedule.add_systems((sync_simple_transforms, propagate_transforms));
+
+        let mut command_queue = CommandQueue::default();
+        let mut commands = Commands::new(&mut command_queue, &world);
+        let root = commands.spawn(offset_transform(3.3)).id();
+        let parent = commands.spawn(offset_transform(4.4)).id();
+        let child = commands.spawn(offset_transform(5.5)).id();
+        commands.entity(parent).set_parent(root);
+        commands.entity(child).set_parent(parent);
+        command_queue.apply(&mut world);
+        schedule.run(&mut world);
+
+        assert_eq!(
+            world.get::<GlobalTransform>(parent).unwrap(),
+            &offset_global_transform(4.4 + 3.3),
+            "The transform systems didn't run, ie: `GlobalTransform` wasn't updated",
+        );
+
+        // Remove parent of `parent`
+        let mut command_queue = CommandQueue::default();
+        let mut commands = Commands::new(&mut command_queue, &world);
+        commands.entity(parent).remove_parent();
+        command_queue.apply(&mut world);
+        schedule.run(&mut world);
+
+        assert_eq!(
+            world.get::<GlobalTransform>(parent).unwrap(),
+            &offset_global_transform(4.4),
+            "The global transform of an orphaned entity wasn't updated properly",
+        );
+
+        // Remove parent of `child`
+        let mut command_queue = CommandQueue::default();
+        let mut commands = Commands::new(&mut command_queue, &world);
+        commands.entity(child).remove_parent();
+        command_queue.apply(&mut world);
+        schedule.run(&mut world);
+
+        assert_eq!(
+            world.get::<GlobalTransform>(child).unwrap(),
+            &offset_global_transform(5.5),
+            "The global transform of an orphaned entity wasn't updated properly",
+        );
+    }
+
     #[test]
     fn did_propagate() {
         ComputeTaskPool::init(TaskPool::default);