fix: AnimationGroup with negative z_index

manim/scene/scene.py

@@ -848,7 +848,18 @@
         # as soon as there's one that needs updating of
         # some kind per frame, return the list from that
         # point forward.
-        animation_mobjects = [anim.mobject for anim in animations]
+
+        # Imported inside the method to avoid cyclic import
+        from ..animation.composition import AnimationGroup
+
+        animation_mobjects = []
+        for anim in animations:
+            if isinstance(anim, AnimationGroup):
+                for sub in anim.animations:
+                    animation_mobjects.append(sub.mobject)
+            else:
+                animation_mobjects.append(anim.mobject)
+
         mobjects = self.get_mobject_family_members()
         for i, mob in enumerate(mobjects):
             update_possibilities = [

tests/test_graphical_units/test_geometry.py

@@ -174,6 +174,21 @@ def test_ZIndex(scene):
     scene.play(ApplyMethod(triangle.shift, 2 * UP))
 
 
+@frames_comparison(last_frame=False)
+def test_negative_z_index_AnimationGroup(scene):
+    # https://github.com/ManimCommunity/manim/issues/3334
+    s = Square().set_z_index(-1)
+    scene.play(AnimationGroup(GrowFromCenter(s)))
+
+
+@frames_comparison(last_frame=False)
+def test_negative_z_index_LaggedStart(scene):
+    # https://github.com/ManimCommunity/manim/issues/3914
+    background = Rectangle(z_index=-1)
+    line = Line(2 * LEFT, 2 * RIGHT, color=RED_D, z_index=-1)
+    scene.play(LaggedStart(FadeIn(background), FadeIn(line), lag_ratio=0.5))
+
+
 @frames_comparison
 def test_Angle(scene):
     l1 = Line(ORIGIN, RIGHT)