Fix wrong value of MObject width/height with empty VGroup

manim/mobject/mobject.py

@@ -1979,13 +1979,13 @@ def restore(self) -> Self:
         self.become(self.saved_state)
         return self
 
-    def reduce_across_dimension(self, reduce_func: Callable, dim: int):
+    def reduce_across_dimension(self, reduce_func: Callable, dim: int) -> float | None:
         """Find the min or max value from a dimension across all points in this and submobjects."""
         assert dim >= 0
         assert dim <= 2
         if len(self.submobjects) == 0 and len(self.points) == 0:
-            # If we have no points and no submobjects, return 0 (e.g. center)
-            return 0
+            # If we have no points and no submobjects, return None
+            return None
 
         # If we do not have points (but do have submobjects)
         # use only the points from those.
@@ -1998,7 +1998,11 @@ def reduce_across_dimension(self, reduce_func: Callable, dim: int):
         # smallest dimension they have and compare it to the return value.
         for mobj in self.submobjects:
             value = mobj.reduce_across_dimension(reduce_func, dim)
-            rv = value if rv is None else reduce_func([value, rv])
+            if rv is None:
+                rv = value
+            else:
+                if value is not None:
+                    rv = reduce_func([value, rv])
         return rv
 
     def nonempty_submobjects(self) -> list[Self]:
@@ -2149,10 +2153,11 @@ def get_nadir(self) -> Point3D:
 
     def length_over_dim(self, dim: int) -> float:
         """Measure the length of an :class:`~.Mobject` in a certain direction."""
-        return self.reduce_across_dimension(
-            max,
-            dim,
-        ) - self.reduce_across_dimension(min, dim)
+        val_max = self.reduce_across_dimension(max, dim)
+        val_min = self.reduce_across_dimension(min, dim)
+        if val_max is None and val_min is None:
+            return 0
+        return val_max - val_min
 
     def get_coord(self, dim: int, direction: Vector3D = ORIGIN):
         """Meant to generalize ``get_x``, ``get_y`` and ``get_z``"""

tests/module/mobject/mobject/test_mobject.py

@@ -3,7 +3,7 @@
 import numpy as np
 import pytest
 
-from manim import DL, UR, Circle, Mobject, Rectangle, Square, VGroup
+from manim import DL, DR, UL, UR, Circle, Mobject, Rectangle, Square, VGroup
 
 
 def test_mobject_add():
@@ -136,21 +136,19 @@ def test_mobject_dimensions_nested_mobjects():
 
 
 def test_mobject_dimensions_mobjects_with_no_points_are_at_origin():
-    rect = Rectangle(width=2, height=3)
-    rect.move_to([-4, -5, 0])
-    outer_group = VGroup(rect)
-
-    # This is as one would expect
-    assert outer_group.width == 2
-    assert outer_group.height == 3
-
-    # Adding a mobject with no points has a quirk of adding a "point"
-    # to [0, 0, 0] (the origin). This changes the size of the outer
-    # group because now the bottom left corner is at [-5, -6.5, 0]
-    # but the upper right corner is [0, 0, 0] instead of [-3, -3.5, 0]
-    outer_group.add(VGroup())
-    assert outer_group.width == 5
-    assert outer_group.height == 6.5
+    for direction in [DL, DR, UL, UR]:
+        rect = Rectangle(width=2, height=3)
+        rect.move_to(direction * 10)
+        outer_group = VGroup(rect)
+
+        # This is as one would expect
+        assert outer_group.width == 2
+        assert outer_group.height == 3
+
+        # Adding a mobject with no points does not change its size
+        outer_group.add(VGroup())
+        assert outer_group.width == 2
+        assert outer_group.height == 3
 
 
 def test_mobject_dimensions_has_points_and_children():