Why is the value of `gl_FragCoord.z` is always 0.5?

Question

I am working on a complicated WebGL program where I have to recover depth in fragment shader.

However, the value of gl_FragCoord.z is always 0.5, quite strange.

// I use this method to verify the value of gl_FragCoord.z
if (gl_FragCoord.z == 0.5) {
    fragColor= vec4(1.0, 0.0, 0.0, 0.5);
} 
else {
    fragColor= vec4(0.0, 0.0, 1.0, 0.5);
}

The depth test is turned on with

gl.enable(gl.DEPTH_TEST);
gl.depthFunc(gl.ALWAYS);

I have seen many similar problems with gl_FragCoord.z valued zero, which is always caused by disable depth test. So I wonder what might cause my problem? Or is there any other way to recover depth of fragment without gl_FragCoord.z?

Thanks!

---

I have launched a issue of the original project I was working on.