custom.js

function getOffset(el) {
  const rect = el.getBoundingClientRect();
  return {
    left: rect.left + window.scrollX,
    top: rect.top + window.scrollY,
  };
}

const prevOffset = {};

const displayEffect = (event) => {
  if (prevOffset.top === undefined) {
    let initialOffset = getOffset(document.getElementsByClassName("cursor")[0]);
    prevOffset.top = initialOffset.top;
    prevOffset.left = initialOffset.left;
  }

  const existingRect = document.getElementById("myRect");
  if (existingRect !== null) {
    existingRect.remove();
  }

  setTimeout(function () {
    const currOffset = getOffset(document.getElementsByClassName("cursor")[0]);
    const top = prevOffset.top;
    const left = prevOffset.left;
    const rect = document.createElement("div");

    // console.log("prev: ", top, left);
    // console.log("curr: ", currOffset.top, currOffset.left);

    rect.id = "myRect";
    rect.style.cssText = `
      position:absolute;
      top:${top}px;
      left:${left}px;
      width:9px;
      height:20px;
      z-index:10;
      background-color:red;
      opacity: 0.5;
    `;
    rect.animate(
      [
        // keyframes
        { transform: "scale(1) skew(10deg)" },
        { transform: "scale(0) skew(10deg)" },
      ],
      {
        // timing options
        duration: 500,
        easing: "ease-in-out",
        direction: "alternate",
        iterations: Infinity,
      }
    );

    document.body.appendChild(rect);
    prevOffset.top = currOffset.top;
    prevOffset.left = currOffset.left;
  }, 24);
};

// NOTE: First enable editor.cursorSmoothCaretAnimation.
// NOTE: Uncomment this to get a cursor trail effect.
document.addEventListener("keydown", displayEffect);