Fix VECTOR/LOCAL transitions in Node3D

Fixes #62225, supersedes #62227

scene/3d/node_3d.h

@@ -52,6 +52,9 @@ class Node3D : public Node {
 	GDCLASS(Node3D, Node);
 
 public:
+	// Edit mode for the rotation.
+	// THIS MODE ONLY AFFECTS HOW DATA IS EDITED AND SAVED
+	// IT DOES _NOT_ AFFECT THE TRANSFORM LOGIC (see comment in TransformDirty).
 	enum RotationEditMode {
 		ROTATION_EDIT_MODE_EULER,
 		ROTATION_EDIT_MODE_QUATERNION,
@@ -68,11 +71,27 @@ public:
 	};
 
 private:
+	// For the sake of ease of use, Node3D can operate with Transforms (Basis+Origin), Quaterinon/Scale and Euler Rotation/Scale.
+	// Transform and Quaterinon are stored in data.local_transform Basis (so quaternion is not really stored, but converted back/forth from 3x3 matrix on demand).
+	// Euler needs to be kept separate because converting to Basis and back may result in a different vector (which is troublesome for users
+	// editing in the inspector, not only because of the numerical precision loss but because they expect these rotations to be consistent, or support
+	// "redundant" rotations for animation interpolation, like going from 0 to 720 degrees).
+	//
+	// As such, the system works in a way where if the local transform is set (via transform/basis/quaternion), the EULER rotation and scale becomes dirty.
+	// It will remain dirty until reading back is attempted (for performance reasons). Likewise, if the Euler rotation scale are set, the local transform
+	// will become dirty (and again, will not become valid again until read).
+	//
+	// All this is transparent from outside the Node3D API, which allows everything to works by calling these functions in exchange.
+	//
+	// Additionally, setting either transform, quaternion, Euler rotation or scale makes the global transform dirty, which will be updated when read again.
+	//
+	// NOTE: Again, RotationEditMode is _independent_ of this mechanism, it is only meant to expose the right set of properties for editing (editor) and saving
+	// (to scene, in order to keep the same values and avoid data loss on conversions). It has zero influence in the logic described above.
 	enum TransformDirty {
 		DIRTY_NONE = 0,
-		DIRTY_VECTORS = 1,
-		DIRTY_LOCAL = 2,
-		DIRTY_GLOBAL = 4
+		DIRTY_EULER_ROTATION_AND_SCALE = 1,
+		DIRTY_LOCAL_TRANSFORM = 2,
+		DIRTY_GLOBAL_TRANSFORM = 4
 	};
 
 	mutable SelfList<Node> xform_change;
@@ -80,8 +99,8 @@ private:
 	struct Data {
 		mutable Transform3D global_transform;
 		mutable Transform3D local_transform;
-		mutable Basis::EulerOrder rotation_order = Basis::EULER_ORDER_YXZ;
-		mutable Vector3 rotation;
+		mutable Basis::EulerOrder euler_rotation_order = Basis::EULER_ORDER_YXZ;
+		mutable Vector3 euler_rotation;
 		mutable Vector3 scale = Vector3(1, 1, 1);
 		mutable RotationEditMode rotation_edit_mode = ROTATION_EDIT_MODE_EULER;
 
@@ -131,6 +150,7 @@ protected:
 	_FORCE_INLINE_ void set_ignore_transform_notification(bool p_ignore) { data.ignore_notification = p_ignore; }
 
 	_FORCE_INLINE_ void _update_local_transform() const;
+	_FORCE_INLINE_ void _update_rotation_and_scale() const;
 
 	void _notification(int p_what);
 	static void _bind_methods();