Changes to SkeletonIK:

* Removed the pointers to PhysicalBone in the code, as they were unused.
* Forward ported the SkeletonIK bone scaling fix I made from Godot 3.2 to Godot 4.0.
* Fixed issue where the root bone in the IK chain would not rotate correctly.
  * The issue turned out to be the update_chain function being called in solve. This would override the root bone transform incorrectly and that would cause it not to rotate after just a single solve. Removing the update_chain function fixes the issue and based on my testing there are no adverse effects.
  * While the old fix on this PR (prior to a force push) required a hack fix, this new fix does not!
* Removed the update_chain function. This change doesn't appear to have any adverse effects in any of the projects I tested (including with animations, Skeleton3D or otherwise, from AnimationPlayer nodes!)
* Fixed issue where the scale of the Skeleton node would change the position of the target, causing it not to work with skeletons that have a global scale of anything but 1.

(cherry picked from commit a622649876374ffef9b92b581c255cc5d29cd203)

scene/animation/skeleton_ik.cpp

@@ -64,7 +64,6 @@ bool FabrikInverseKinematic::build_chain(Task *p_task, bool p_force_simple_chain
 	chain.chain_root.bone = p_task->root_bone;
 	chain.chain_root.initial_transform = p_task->skeleton->get_bone_global_pose(chain.chain_root.bone);
 	chain.chain_root.current_pos = chain.chain_root.initial_transform.origin;
-	chain.chain_root.pb = p_task->skeleton->get_physical_bone(chain.chain_root.bone);
 	chain.middle_chain_item = NULL;
 
 	// Holds all IDs that are composing a single chain in reverse order
@@ -101,8 +100,6 @@ bool FabrikInverseKinematic::build_chain(Task *p_task, bool p_force_simple_chain
 
 				child_ci = sub_chain->add_child(chain_ids[i]);
 
-				child_ci->pb = p_task->skeleton->get_physical_bone(child_ci->bone);
-
 				child_ci->initial_transform = p_task->skeleton->get_bone_global_pose(child_ci->bone);
 				child_ci->current_pos = child_ci->initial_transform.origin;
 
@@ -136,20 +133,6 @@ bool FabrikInverseKinematic::build_chain(Task *p_task, bool p_force_simple_chain
 	return true;
 }
 
-void FabrikInverseKinematic::update_chain(const Skeleton *p_sk, ChainItem *p_chain_item) {
-
-	if (!p_chain_item)
-		return;
-
-	p_chain_item->initial_transform = p_sk->get_bone_global_pose(p_chain_item->bone);
-	p_chain_item->current_pos = p_chain_item->initial_transform.origin;
-
-	ChainItem *items = p_chain_item->children.ptrw();
-	for (int i = 0; i < p_chain_item->children.size(); i += 1) {
-		update_chain(p_sk, items + i);
-	}
-}
-
 void FabrikInverseKinematic::solve_simple(Task *p_task, bool p_solve_magnet) {
 
 	real_t distance_to_goal(1e4);
@@ -297,9 +280,7 @@ void FabrikInverseKinematic::solve(Task *p_task, real_t blending_delta, bool ove
 		p_task->skeleton->set_bone_global_pose_override(p_task->chain.tips[i].chain_item->bone, Transform(), 0.0, true);
 	}
 
-	make_goal(p_task, p_task->skeleton->get_global_transform().affine_inverse().scaled(p_task->skeleton->get_global_transform().get_basis().get_scale()), blending_delta);
-
-	update_chain(p_task->skeleton, &p_task->chain.chain_root);
+	make_goal(p_task, p_task->skeleton->get_global_transform().affine_inverse(), blending_delta);
 
 	if (p_use_magnet && p_task->chain.middle_chain_item) {
 		p_task->chain.magnet_position = p_task->chain.middle_chain_item->initial_transform.origin.linear_interpolate(p_magnet_position, blending_delta);
@@ -323,6 +304,7 @@ void FabrikInverseKinematic::solve(Task *p_task, real_t blending_delta, bool ove
 				const real_t rot_angle(Math::acos(CLAMP(initial_ori.dot(ci->current_ori), -1, 1)));
 				new_bone_pose.basis.rotate(rot_axis, rot_angle);
 			}
+
 		} else {
 			// Set target orientation to tip
 			if (override_tip_basis)

scene/animation/skeleton_ik.h

@@ -54,7 +54,6 @@ class FabrikInverseKinematic {
 
 		// Bone info
 		BoneId bone;
-		PhysicalBone *pb;
 
 		real_t length;
 		/// Positions relative to root bone
@@ -66,7 +65,6 @@ class FabrikInverseKinematic {
 		ChainItem() :
 				parent_item(NULL),
 				bone(-1),
-				pb(NULL),
 				length(0) {}
 
 		ChainItem *find_child(const BoneId p_bone_id);
@@ -121,8 +119,6 @@ private:
 	/// Init a chain that starts from the root to tip
 	static bool build_chain(Task *p_task, bool p_force_simple_chain = true);
 
-	static void update_chain(const Skeleton *p_sk, ChainItem *p_chain_item);
-
 	static void solve_simple(Task *p_task, bool p_solve_magnet);
 	/// Special solvers that solve only chains with one end effector
 	static void solve_simple_backwards(Chain &r_chain, bool p_solve_magnet);