Basic Fragment Shader UV offset

I'm trying to get a handle on the basic of working with texture UVs in a fragment shader. I have the following code:

 float4 frag (v2f i) : SV_Target
 {
      float new_UV_value = (uv.x) * offsetValue;
      uv.x -= new_UV_value;
             
      return uv.x;
 }

Now, offsetValue is a property on the shader, and has the range from -1 to 0. When offsetValue is 0, the resulting image looks like this, a nice smooth gradient.

As offsetValue gets decreased to -1, I assumed that the entire gradient would be squeezed into the left side of the square.

However, when the shader evaluates offsetValue at -1, I get this instead.

So I know I'm doing something wrong, and I assume it's how I'm working with the uv coordinates. Any suggestions on how to fix this would be greatly appreciated.