Fix collider insertion/removal tracking.

src/dynamics/rigid_body.rs

@@ -167,12 +167,10 @@ impl RigidBody {
 
     /// Adds a collider to this rigid-body.
     pub(crate) fn add_collider(&mut self, handle: ColliderHandle, coll: &Collider) {
-        if !self.changes.contains(RigidBodyChanges::MODIFIED) {
-            self.changes.set(
-                RigidBodyChanges::MODIFIED | RigidBodyChanges::COLLIDERS,
-                true,
-            );
-        }
+        self.changes.set(
+            RigidBodyChanges::MODIFIED | RigidBodyChanges::COLLIDERS,
+            true,
+        );
 
         let mass_properties = coll
             .mass_properties()
@@ -193,6 +191,7 @@ impl RigidBody {
     /// Removes a collider from this rigid-body.
     pub(crate) fn remove_collider_internal(&mut self, handle: ColliderHandle, coll: &Collider) {
         if let Some(i) = self.colliders.iter().position(|e| *e == handle) {
+            self.changes.set(RigidBodyChanges::COLLIDERS, true);
             self.colliders.swap_remove(i);
             let mass_properties = coll
                 .mass_properties()

src/geometry/collider_set.rs

@@ -63,8 +63,11 @@ impl ColliderSet {
         coll.reset_internal_references();
 
         coll.parent = parent_handle;
+
+        // NOTE: we use `get_mut` instead of `get_mut_internal` so that the
+        // modification flag is updated properly.
         let parent = bodies
-            .get_mut_internal(parent_handle)
+            .get_mut(parent_handle)
             .expect("Parent rigid body not found.");
         coll.position = parent.position * coll.delta;
         coll.predicted_position = parent.predicted_position * coll.delta;
@@ -89,7 +92,9 @@ impl ColliderSet {
         /*
          * Delete the collider from its parent body.
          */
-        if let Some(parent) = bodies.get_mut_internal(collider.parent) {
+        // NOTE: we use `get_mut` instead of `get_mut_internal` so that the
+        // modification flag is updated properly.
+        if let Some(parent) = bodies.get_mut(collider.parent) {
             parent.remove_collider_internal(handle, &collider);
 
             if wake_up {