Can't seem to draw a vertex_list in Pyglet 2

I am trying to familiarize myself with the graphics system in Pyglet 2. I have a fair knowledge of older versions of OpenGL. I can't find any sample code that is up to date. I built the following code by stuffing together bits from the documentation. It is supposed to produce a window with a color triangle in the lower left quadrant.

It produces a blank window (probably the least helpful failure in OpenGL).

import pyglet
from pyglet.gl import *
from pyglet.math import *
from pyglet.graphics.shader import Shader, ShaderProgram


class PictureWindow(pyglet.window.Window):
    
    def __init__(self):
        super().__init__(800, 600,
                         resizable=True)

        self.set_minimum_size(320, 200)

    def on_draw(self):
        self.clear()
        batch.draw()
        
    def on_resize(self, width, height):
        glViewport(0, 0, *self.get_framebuffer_size())
        window.projection = Mat4.orthogonal_projection(0, width, 0, height, -255, 255)
        return pyglet.event.EVENT_HANDLED


window = PictureWindow()    # This must be the first graphics related call.

batch = pyglet.graphics.Batch()

# Define the GLSL Shaders.
vertex_source = """#version 150 core
    in vec2 position;
    in vec4 colors;
    out vec4 vertex_colors;

    uniform mat4 projection;

    void main()
    {
        gl_Position = projection * vec4(position, 0.0, 1.0);
        vertex_colors = colors;
    }
"""

fragment_source = """#version 150 core
    in vec4 vertex_colors;
    out vec4 final_color;

    void main()
    {
        final_color = vertex_colors;
    }
"""

vert_shader = Shader(vertex_source, 'vertex')
frag_shader = Shader(fragment_source, 'fragment')
program = ShaderProgram(vert_shader, frag_shader)

vlist = program.vertex_list(3, pyglet.gl.GL_TRIANGLES, batch=batch)

vlist.position[:] = (100, 300,
                     200, 250,
                     200, 350)

vlist.colors[:] = (1.0, 0.0, 0.0, 1.0,
                   0.0, 1.0, 0.0, 1.0,
                   0.0, 0.0, 1.0, 1.0)


if __name__ == '__main__':
    pyglet.app.run(interval=1/60)

It runs fine - no errors, but gives me an empty window. So, I figure I am missing a step, but I can't find it in the docs.

I have tried using integer colors as well - no change. I have also tried various Opacity (A) values in the colors.

It is acting like it is rendering out of the field of view, but if I am reading the projection function correctly, it should be in the lower left quadrant.

You must manually change the value of the corresponding uniform in the shader program when you change the projection matrix:

class PictureWindow(pyglet.window.Window):
    # [...]

        def on_resize(self, width, height):
        glViewport(0, 0, *self.get_framebuffer_size())

        projection_matrix = Mat4.orthogonal_projection(0, width, 0, height, -255, 255)
        window.projection = projection_matrix
        program.uniforms['projection'].set(projection_matrix)

        return pyglet.event.EVENT_HANDLED