JSFiddle - React, Tailwind, and code Playground

HTML

<div id="div1"> Text1 </div>
 <div id="div2"> Text2 </div>
 <div id="div3"> Text3 </div>

JavaScript

function getSelectedText() {
  var t = '';
  if(window.getSelection){
    t = window.getSelection();
  }
  else if(document.getSelection){
    t = document.getSelection();
  }
  else if(document.selection){
    t = document.selection.createRange();
  }  
  if(t) 
      return {
          selectedText:t.text ? t.text : t.toString(),
          fullContent:t.anchorNode ? t.anchorNode.nodeValue : t.parentElement().innerHTML
      };
}


$(function(){
    $(document).on('mouseup', function(){
        var selection=getSelectedText();
        if(selection.selectedText)
        {
           var selected = $.trim(selection.selectedText);
           var full = $.trim(selection.fullContent);
            if(selected!=full)
               alert('Full content is not selected');
           else
               alert('Full content is selected');
            
        }
    });
});