add zorder to mobject

manimlib/mobject/mobject.py

@@ -83,6 +83,7 @@ class Mobject(object):
         # If true, the mobject will not get rotated according to camera position
         is_fixed_in_frame: bool = False,
         depth_test: bool = False,
+        zorder: int = 0,
     ):
         self.color = color
         self.opacity = opacity
@@ -90,6 +91,8 @@ class Mobject(object):
         self.texture_paths = texture_paths
         self._is_fixed_in_frame = is_fixed_in_frame
         self.depth_test = depth_test
+        self.zorder = zorder
+        self._scene_order = 0
 
         # Internal state
         self.submobjects: list[Mobject] = []

manimlib/scene/scene.py

@@ -414,7 +414,13 @@ class Scene(object):
         foreground in the order with which they are added.
         """
         self.remove(*new_mobjects)
+        idx = 0
+        scene_order = len(self.mobjects)
+        for m in new_mobjects:
+            m._scene_order = scene_order+idx
+            idx += 1
         self.mobjects += new_mobjects
+        self.mobjects = [self.mobjects[0]]+sorted(self.mobjects[1:], key=lambda m:(m.zorder,m._scene_order))
         self.id_to_mobject_map.update({
             id(sm): sm
             for m in new_mobjects