OpenGL ES Shading Language - Praise for OpenGL ES 30 Programming Guide - page 122

The same multipass vertex shaders using invariance for position produce
the correct image in Figure 5-2.

Figure 5-2 Z Fighting Avoided Using Invariance

The introduction of invariance gives the shader writer a way to specify
that if the same computations are used to compute an output, its value
must be exactly the same (or invariant). The invariant keyword can be
used either on varying declarations or for varyings that have already been
declared. Some examples follow:

invariant gl_Position;
invariant texCoord;

Once invariance is declared for an output, the compiler guarantees
that the results will be the same given the same computations and
inputs into the shader. For example, given two vertex shaders that
compute output position by multiplying the view projection matrix
by the input position, you are guaranteed that those positions will be
invariant.

#version 300 es
uniform mat4 u_viewProjMatrix;
layout(location = 0) in vec4 a_vertex;
invariant gl_Position;