Forbid "incoherent" selection in a document (selection that can not be expressed as a new tag in the DOM tree)

coherent_selection.js

document.onmousemove = function(evt) {
  var s = window.getSelection();
  if (s.rangeCount === 0) return;
  while(s.rangeCount > 1) s.removeRange(s.getRangeAt(1));
  r = s.getRangeAt(0);
  if (r.startContainer.parentNode !== r.endContainer.parentNode) {
    r.setEnd(r.startContainer, r.startContainer.length);
    evt.preventDefault();
  }
}