JSFiddle - React, Tailwind, and code Playground

HTML

<div contenteditable="true" class="js_wd_gen_ucclass idmrTinyMCE idmr-context-menu mce-content-body"><table border="5"><tbody><tr height="18"><td width="129"><br></td><td width="130"><br></td><td width="131"><br></td><td width="132"><br></td></tr><tr height="18"><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr height="18"><td><br></td><td><br></td><td><br></td><td><br></td></tr></tbody></table></div>

CSS

div {
  min-height: 65px; 
  width: 100%; 
  background-color: rgb(224, 224, 248);
}

table {
  border-collapse: collapse;
}

JavaScript

$('.js_wd_gen_ucclass').live('focusout', function (){
    $(this).find('table').each(function () {
          var ele, actualWidth, totalWidth;
          
          totalWidth = 0;
          ele = $(this);
          // Get the border width for the last table width operation
          var border = parseInt(ele.attr("border"));

          ele.find('tr:first td').each(function () {
          	// Get outerWidth instead of Width, so manage borders
            var actualWidth = $(this).outerWidth();
            $(this).attr("width", actualWidth);
            // Compute sum of total width the cell and one border
            totalWidth = totalWidth + actualWidth;
          });
          // Set Table width and add one border width 
          // Because  'number of border = number of cell + 1'
          ele.attr("width", totalWidth + border);
  });
});