Code for reproducing a suspected pyglet PNG saving bug.

piglet_save_png_bug.py

#!/usr/bin/env python3                                                                                   

import pyglet


class RenderText(pyglet.window.Window):
    def __init__(self, text, width=512, height=256):
        super().__init__(width=width, height=height)
	pyglet.gl.glClearColor(0, 0, 0, 1)  # Fixes the problem.
	self.document = pyglet.text.decode_text(text)
	self.layout = pyglet.text.layout.TextLayout(
            self.document, width, height, multiline=True
        )
        self.document.set_style(
            start=0,
            end=0,
            attributes={
		"font_size": 12,
                "color": (255, 255, 255, 255),
                "font_name": "Source Code Pro",
            },
	)

    def on_draw(self):
	self.clear()
	self.layout.draw()
        pyglet.image.get_buffer_manager().get_color_buffer().save("output.png")


if __name__ == "__main__":
    text = """Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt \
ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nis\
i ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cil\
lum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui o\
fficia deserunt mollit anim id est laborum."""

    window = RenderText(text)
    pyglet.app.run()

solved by adding to the __init__ of my Window descended class:

pyglet.gl.glClearColor(0, 0, 0, 1)