Today's shader sketch was working on an effect. It took me way too long to figure out this code. &sol

    vec2 st = gl_FragCoord.xy / u_resolution.xy;
    int iters = 32;
    float ratio = 0.5 + 0.5*sin(0.5*u_time);
    for (int i = 0; i < iters; i++) {
      if (st.x < ratio) {
        st.x = st.x / ratio;
        break;
      } else {
        float temp = st.x - ratio;
        st.x = (1.0 - st.y);
        st.y = temp / (1.0-ratio);
      }
    }