setDegreeByClientPosition = (clientX, clienY) => {
    const opposite = -(clienY - $state.y);
    const adjacent = clientX - $state.x;
    let degree = Math.atan(opposite / adjacent) * 360 / tPI;

    if (adjacent < 0) {
      degree += 180;
    }
    
    state.degree = degree;
    
    // Force draw to avoid glitches!
    draw(true);
  }
Calculate and set degree from mouse position.