Почему рисует ниже курсора

     const canvas = document.getElementById("contract-paint") 
    canvas.height = window.innerHeight 
    canvas.width = window.innerWidth 
 
    const ctx = canvas.getContext("2d") 
 
    // previous mouse positions 
    // They will be null initially 
    let prevX = null 
    let prevY = null 
 
    // How thick the lines should be 
    ctx.lineWidth = 5 
 
    window.addEventListener("mousemove", (e) => { 
      // initially previous mouse positions are null 
      // so we can't draw a line 
      if (prevX == null || prevY == null) { 
        // Set the previous mouse positions to the current mouse positions 
        prevX = e.clientX 
        prevY = e.clientY 
        return 
      } 
 
      // Current mouse position 
      let currentX = e.clientX 
      let currentY = e.clientY 
 
      // Drawing a line from the previous mouse position to the current mouse position 
      ctx.beginPath() 
      ctx.moveTo(prevX, prevY) 
      ctx.lineTo(currentX, currentY) 
      ctx.stroke() 
 
      // Update previous mouse position 
      prevX = currentX 
      prevY = currentY 
    });