File: selection_capture.js

function getSelectionText() {
    var text = "";
    if (window.getSelection) {
        text = window.getSelection().toString();
    } else if (document.selection && document.selection.type != "Control") {
        text = document.selection.createRange().text;
    }
    return text;
}

document.onmouseup = document.onkeyup = document.onselectionchange = function() {
  document.getElementById("sel").value = getSelectionText();
};