Unity resolution problem, mesh has holes when zooming out

Question

I have a server which renders a mesh using OpenGL with a custom frame buffer. In my fragment shader I write the gl_primitiveID into an output variable. Afterwards I call glReadPixels to read the IDs out. Then I know which triangles were rendered (and are therefore visible) and I send all those triangles to a client which runs on Unity. On this client I add the vertex and index data to a GameObject and it renders it without a problem. I get the exact same rendering result in Unity as I got with OpenGL, unless I start to zoom out.

Here are pictures of the mesh rendered with Unity:

My first thought was that I have different resolutions, but this is not the case. I have 1920*1080 on both server and client. I use the same view and projection matrix from the client on my server, so this also shouldn't be the problem. What could be cause of this error?

In case you need to see some of the code I wrote.

Here is my vertex shader code:

#version 330 core
layout (location = 0) in vec3 position;

uniform mat4 view;
uniform mat4 projection;

void main()
{
   gl_Position = projection * view * vec4(position, 1.0f);
}

Here is my fragment shader code:

#version 330 core
layout(location = 0) out int primitiveID;

void main(void)
{ 
   primitiveID = gl_PrimitiveID + 1; // +1 because the first primitive is 0
}

and here is my getVisibleTriangles method:

std::set<int> getVisibleTriangles() {
    glReadBuffer(GL_COLOR_ATTACHMENT0);
    glReadPixels(0, 0, SCR_WIDTH, SCR_HEIGHT, GL_RED_INTEGER, GL_INT, &pixels[0]);

    std::set<int> visibleTriangles;
    for (int i = 0; i < pixelsLength; i += 4) {
        int id = * (int*) &pixels[i];
        if (id != 0) {
            visibleTriangles.insert(id-1); // id 0 is NO_PRIMITIVE
        }
    }
    return visibleTriangles;
}

Answer 1

Oh my god, I can't believe I made such a stupid mistake. After all it WAS a resolution problem.

I didn't call gl_Viewport (only when resizing the window). Apparently when creating a window with glfwCreateWindow GLFW creates the window but since the parameters are only hints and not hard constraints (as it is stated here: glfw reference) it is possible that those are not exactly fulfilled. Since I have passed a desired size of 1920*1080 (which is also my resolution) apparently the drawing area did not actually get the size of 1920*1080, because there is also some space needed for the menu etc. So it is rendered with a lower resolution (to be exact, it was 1920*1061) on the server which results in missing triangles on the client.