Result

マウスドラッグした文字数を数えます。JSはバニラです
使う機会があるかは分かりません

js

function biR() {
   document.addEventListener("mousedown", rOS);
   document.addEventListener("mouseup", rOS);
   document.addEventListener("onkeyup", rOS);
   document.addEventListener("onkeydown", rOS);
   document.addEventListener("onkeydown", rOS);
   document.onselectionchange = function() {
     rOS();
   };
 }

 function rOS() {
   var cout, rts, rt;
   c = window.getSelection().toString().length
   rts = document.getElementsByClassName('cR');
   for (var i = 0; i < rts.length; i++) {
     rt = rts[i];

     for (var i = 0; i < rt.childNodes.length; i++) {
       if (rt.childNodes[i].className == "cC") {
         rt.childNodes[i].innerText = c;
       }
     }


     if (c > 0) {
       rt.classList.remove("df");
     } else {
       rt.classList.add("df");
     }
   }
 }

 function stopTest() {
   clearInterval(test);
 }

 biR();

via

Lorem Ipsum Tools