Integration with moderngl

[oogre]

Hello,
I'm trying to integrate syphonpy with moderGL,
the goal is to build a modular video synthetizer.

I think I miss something, cause my understanding of syphonpy and modernGL is very partial.
First of all, I test both of these libraries, they are running well.
After that I mixed exemples form A and B, here lies the mix :

import numpy as np
import syphonpy
import pprint
from PIL import Image
import moderngl
from ported._example import Example
import time as delay

class AlphaBlending(Example):
    gl_version = (3, 3)
    title = "Alpha Blending"

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.id = 0;
        self.server = syphonpy.SyphonServer("headlessTest2")
        self.size = (1280, 720)
        self.fbo = self.ctx.framebuffer(
            color_attachments=[self.ctx.texture(self.size, 4)]
        )
        self.fbo.use()
        self.prog = self.ctx.program(
            vertex_shader='''
                #version 330

                in vec2 vert;

                in vec4 vert_color;
                out vec4 frag_color;

                uniform vec2 scale;
                uniform float rotation;

                void main() {
                    frag_color = vert_color;
                    float r = rotation * (0.5 + gl_InstanceID * 0.05);
                    mat2 rot = mat2(cos(r), sin(r), -sin(r), cos(r));
                    gl_Position = vec4((rot * vert) * scale, 0.0, 1.0);
                }
            ''',
            fragment_shader='''
                #version 330
                in vec4 frag_color;
                out vec4 color;
                void main() {
                    color = vec4(frag_color);
                }
            ''',
        )

        self.scale = self.prog['scale']
        self.rotation = self.prog['rotation']

        self.scale.value = (0.5, self.aspect_ratio * 0.5)

        vertices = np.array([
            1.0, 0.0,
            1.0, 0.0, 0.0, 0.5,

            -0.5, 0.86,
            0.0, 1.0, 0.0, 0.5,

            -0.5, -0.86,
            0.0, 0.0, 1.0, 0.5,
        ], dtype='f4')

        self.vbo = self.ctx.buffer(vertices)
        self.vao = self.ctx.simple_vertex_array(self.prog, self.vbo, 'vert', 'vert_color')

    def render(self, time: float, frame_time: float):
        self.ctx.clear(1.0, 1.0, 1.0)
        self.ctx.enable(moderngl.BLEND)
        self.fbo.use()
        self.fbo.clear(255, 255, 255, 255)
        self.rotation.value = time
        self.vao.render(instances=10)

        viewPort = syphonpy.MakeRect(0,0,self.fbo.width,self.fbo.height)
        size = syphonpy.MakeSize(self.fbo.width,self.fbo.height)

        self.server.publish_frame_texture(self.fbo.glo, viewPort, size, False)


if __name__ == '__main__':
    AlphaBlending.run()

I run

python3 basic_alpha_blending.py

it works... the video output is piped to syphon which is what I need, the window build by moderGL is full white.
but I want to run it headless. so

I run

python3 basic_alpha_blending.py --window headless

it does not work. the syphon server looks to be created, but nothing is displayed into syphon recorder
(which I use to visualize the rendering)
later as debug, I saved the rendering FBO to a png file to isolate the bug origin,
this worked without any troubble,
So I imagine the trouble comes from syphonpy
but I'm kind of blocked...

does anyone here ve a bit of advices ?
Thanks for reading me.