Fix empty submobjects distorting width/height of parent Mobject (#4088)

* Issue-4087 Add bug test

* Issue-4087 Fix bug

* Issue-4087 Fix comment and add type hint

* Issue-4087 Fix comment

* Issue-4087 Improve previous test instead of adding a new one

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* Update tests/module/mobject/mobject/test_mobject.py

* Enhance docstring for reduce_across_dimension method

Updated docstring for reduce_across_dimension method to clarify its purpose and parameters.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Remove float typehints for coords in length_over_dim

* Modify test for empty VMobject dimensions

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: Francisco Manríquez Novoa <49853152+chopan050@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

manim/mobject/mobject.py

@@ -2162,13 +2162,31 @@ class Mobject:
 
     def reduce_across_dimension(
         self, reduce_func: Callable[[Iterable[float]], float], dim: int
-    ) -> float:
-        """Find the min or max value from a dimension across all points in this and submobjects."""
+    ) -> float | None:
+        """Find the min or max value from a dimension across all points in this Mobject and its
+        submobjects. This allows for using :meth:`~.length_over_dim` to calculate its length over
+        a dimension, i.e. its height, width or depth. If this Mobject is empty, return ``None``,
+        since this Mobject should not be taken into account when calculating lengths.
+
+        Parameters
+        ----------
+        reduce_func
+            The reducer function to use in order to calculate a value over a dimension.
+        dim
+            The dimension to use. It should be 0, 1 or 2, representing the X, Y or Z coordinate,
+            respectively.
+
+        Returns
+        -------
+        float | None
+            The min or max value over the dimension specified by ``dim``, or ``None`` if this
+            Mobject is empty.
+        """
         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.
@@ -2181,8 +2199,10 @@ class Mobject:
         # 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])
-        assert rv is not None
+            if rv is None:
+                rv = value
+            elif value is not None:
+                rv = reduce_func([value, rv])
         return rv
 
     def nonempty_submobjects(self) -> Sequence[Mobject]:
@@ -2336,11 +2356,10 @@ class Mobject:
 
     def length_over_dim(self, dim: int) -> float:
         """Measure the length of an :class:`~.Mobject` in a certain direction."""
-        max_coord: float = self.reduce_across_dimension(
-            max,
-            dim,
-        )
-        min_coord: float = self.reduce_across_dimension(min, dim)
+        max_coord = self.reduce_across_dimension(max, dim)
+        min_coord = self.reduce_across_dimension(min, dim)
+        if max_coord is None or min_coord is None:
+            return 0
         return max_coord - min_coord
 
     def get_coord(self, dim: int, direction: Vector3DLike = ORIGIN) -> float:

tests/module/mobject/mobject/test_mobject.py

@@ -3,7 +3,20 @@ from __future__ import annotations
 import numpy as np
 import pytest
 
-from manim import DL, PI, UR, Circle, Mobject, Rectangle, Square, Triangle, VGroup
+from manim import (
+    DL,
+    DR,
+    PI,
+    UL,
+    UR,
+    Circle,
+    Mobject,
+    Rectangle,
+    Square,
+    Triangle,
+    VGroup,
+    VMobject,
+)
 
 
 def test_mobject_add():
@@ -135,22 +148,24 @@ def test_mobject_dimensions_nested_mobjects():
     assert is_close(vg.depth, 0.775), vg.depth
 
 
-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)
+def test_mobject_dimensions_mobjects_with_no_points():
+    empty_mob = VMobject()
+    assert empty_mob.width == 0
+    assert empty_mob.height == 0
 
-    # This is as one would expect
-    assert outer_group.width == 2
-    assert outer_group.height == 3
+    for direction in [DL, DR, UL, UR]:
+        rect = Rectangle(width=2, height=3)
+        rect.move_to(direction * 10)
+        outer_group = VGroup(rect)
 
-    # 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
+        # This is as one would expect
+        assert outer_group.width == 2
+        assert outer_group.height == 3
+
+        # Adding a submobject with no points does not change the group size
+        outer_group.add(empty_mob)
+        assert outer_group.width == 2
+        assert outer_group.height == 3
 
 
 def test_mobject_dimensions_has_points_and_children():