Mobject Ordering Regression in 0.15

[sudgy]

Consider the following scene:

class Test(Scene):
    def construct(self):
        a = Square(color = RED)
        b = Square(color = BLUE)
        b.shift(RIGHT)
        self.add(a)
        self.wait()
        self.bring_to_back(b)
        self.play(FadeIn(b))
        self.wait()

In 0.14, this is the output:

Test.mp4

In 0.15, this is the output:

Test.mp4

As you can see, the blue square is correctly drawn behind the red one in 0.14, but in 0.15, the blue square is drawn on top.

[behackl]

Bisecting shows that this was introduced via #2396. This should be fixed by checking whether the mobject belonging to an animation with introducer=True set (like FadeIn) has already been added to the scene.

There is one workaround I can offer: in your example, either put

self.play(FadeIn(b), introducer=False)

or, more verbose,

anim = FadeIn(b)
anim.introducer = False
self.play(anim)

(the obvious FadeIn(b, introducer=False) unfortunately won't work due to the keyword argument not being propagated correctly.)