opengl not respecting z_index order when rendering

[NielsKornerup]

Description of bug / unexpected behavior

When rendering with opengl, mobjects are layered based on the order that they were added rather than the intended z_index order.

Expected behavior

Objects should be ordered according to their z_index

How to reproduce the issue

Run the below code with rendering from either either Cairo or openGL. Cairo gives you the red rectangle while openGL gives you the blue rectangle.

Code for reproducing the problem

config.background_color = WHITE
class MakeRectangles(Scene):
    def construct(self):
        rect1 = Rectangle(fill_color=BLUE, fill_opacity=1)
        rect2 = Rectangle(fill_color=RED, fill_opacity=1)
        rect2.z_index = 2
        rect1.z_index = 1
        self.add(rect2)
        self.add(rect1)

Additional media files

Images/GIFs

openGL output

Cairo output

System specifications

System Details

Python 3.12.3
Manim Community v0.18.1

LaTeX details

Additional comments

This seems to be a problem with the render function here:

https://github.com/ManimCommunity/manim/blob/dbad8a86bc6041af78d5fb7d7a07f76d9b164072/manim/scene/scene.py#L440C1-L449C14