Warning: Extension 'GL_ARB_fragment_coord_conventions' not supported?

[PowerNow]

Hi,
I have 2 visual objects, the 1st contains a background image, the 2nd overlays this with information. Now I need for each pixel in the fragment shader of the 2nd, the underlying color value of the 1st object. The construction is done with Qt Qml, (1st object = image, 2nd object a qquickframebufferobject) and I use defeered rendering.

https://stackoverflow.com/questions/58978147/how-do-get-the-pixel-color-of-the-underlying-object-inside-my-offscreen-framebuf/59218084#59218084

Does someone know why I always get in the fragmend shader

"Warning: Extension 'GL_ARB_fragment_coord_conventions' not supported"

if I want to use GL_EXT_shader_framebuffer_fetch? Subsequently the FS does not compile.

#version 310 es
#extension GL_EXT_shader_framebuffer_fetch : require
precision highp float;
inout vec4 fragColor;
void main()
{
    fragColor = fragColor * 0.5;
}

I checked the extensions via app on different smartphones with opengl es 3.1+ and found GL_EXT_shader_framebuffer_fetch. Propably the GL_ARB is already part of the core package.

Here from my console:

W Camara  : QOpenGLShader::compile(Fragment): 1:2: P0003: Warning: Extension 'GL_ARB_fragment_coord_conventions' not supported
W Camara  : 4:1: P0003: Extension 'GL_EXT_shader_framebuffer_fetch' not supported
W Camara  : *** Problematic Fragment shader source code ***
W Camara  : #version 300 es
W Camara  : #ifdef GL_KHR_blend_equation_advanced
W Camara  : #extension GL_ARB_fragment_coord_conventions : enable
W Camara  : #extension GL_KHR_blend_equation_advanced : enable
W Camara  : #endif
W Camara  : #ifndef GL_FRAGMENT_PRECISION_HIGH
W Camara  : #define highp mediump
W Camara  : #endif
W Camara  : #line 1
W Camara  : #extension GL_EXT_shader_framebuffer_fetch : require

What I'm doing wrong?