BUG: CompositeGenericTransform.contains_branch_separately only checks second component, ignoring first #32099

import matplotlib.transforms as mtransforms
from matplotlib.transforms import Transform, BlendedGenericTransform

# Two independent non-affine transforms
class NonAffine(Transform):
    input_dims = 2
    output_dims = 2
    is_separable = True
    is_affine = False
    def transform_non_affine(self, values): return values

tn1 = NonAffine()
tn2 = NonAffine()

# Blended transform: _x=tn1, _y=tn2
blend = BlendedGenericTransform(tn1, tn2)

# Composite: _a=blend, _b=tn1
composite = blend + tn1

# tn1 is a subtree of _a.x, but NOT of _b
x, y = composite.contains_branch_separately(tn1)
print(f'x={x}, y={y}')  # Got: False, False. Expected: True, False

def contains_branch_separately(self, other_transform):
    if self.output_dims != 2:
        raise ValueError('contains_branch_separately only supports '
                         'transforms with 2 output dimensions')
    if self == other_transform:
        return (True, True)
    return self._b.contains_branch_separately(other_transform)  # <-- only _b!

def contains_branch_separately(self, other_transform):
    if self.output_dims != 2:
        raise ValueError('contains_branch_separately only supports '
                         'transforms with 2 output dimensions')
    if self == other_transform:
        return (True, True)
    a_result = self._a.contains_branch_separately(other_transform)
    b_result = self._b.contains_branch_separately(other_transform)
    return (a_result[0] or b_result[0], a_result[1] or b_result[1])