Did you figure it out?

openfl_TextureCoordv is supposed to be 0 = left, 1 = right, but if you make the window big and then small, its more like 0 = left and 0.5 = right.

Before:

During:

After:

Shader is gl_FragColor = vec4(openfl_TextureCoordv.x, openfl_TextureCoordv.x, openfl_TextureCoordv.x, 1);