Anyone else have an idea why the edge of the white box is curved instead of straight?

So the problem is not that the white square is warped, the problem is that the edges of the white square are curved (non-linear). Based on my shader it seems like the edges of the square should be linear, not slightly curved?

I'm trying to create a shader that will warp my output by by scaling the x axis based on the y position of the pixel. It's working but the sides of the output image are curved instead of straight? Any ideas what I'm doing wrong? Here is the shader and the input and output images are attached. vec4 e...