Get selected text position and place an element next to it

You could position a marker span at the end of the selection, get its coordinates using jQuery, place your button at those coordinates and remove the marker span.

The following should get you started:

var markSelection = (function() {
    var markerTextChar = "\ufeff";
    var markerTextCharEntity = "";

    var markerEl, markerId = "sel_" + new Date().getTime() + "_" + Math.random().toString().substr(2);

    var selectionEl;

    return function(win) {
        win = win || window;
        var doc = win.document;
        var sel, range;
        // Branch for IE <= 8 
        if (doc.selection && doc.selection.createRange) {
            // Clone the TextRange and collapse
            range = doc.selection.createRange().duplicate();
            range.collapse(false);

            // Create the marker element containing a single invisible character by creating literal HTML and insert it
            range.pasteHTML('<span id="' + markerId + '" style="position: relative;">' + markerTextCharEntity + '</span>');
            markerEl = doc.getElementById(markerId);
        } else if (win.getSelection) {
            sel = win.getSelection();
            range = sel.getRangeAt(0).cloneRange();
            range.collapse(false);

            // Create the marker element containing a single invisible character using DOM methods and insert it
            markerEl = doc.createElement("span");
            markerEl.id = markerId;
            markerEl.appendChild( doc.createTextNode(markerTextChar) );
            range.insertNode(markerEl);
        }

        if (markerEl) {
            // Lazily create element to be placed next to the selection
            if (!selectionEl) {
                selectionEl = doc.createElement("div");
                selectionEl.style.border = "solid darkblue 1px";
                selectionEl.style.backgroundColor = "lightgoldenrodyellow";
                selectionEl.innerHTML = "&lt;- selection";
                selectionEl.style.position = "absolute";

                doc.body.appendChild(selectionEl);
            }

            // Find markerEl position http://www.quirksmode.org/js/findpos.html
        var obj = markerEl;
        var left = 0, top = 0;
        do {
            left += obj.offsetLeft;
            top += obj.offsetTop;
        } while (obj = obj.offsetParent);

            // Move the button into place.
            // Substitute your jQuery stuff in here
            selectionEl.style.left = left + "px";
            selectionEl.style.top = top + "px";

            markerEl.parentNode.removeChild(markerEl);
        }
    };
})();

You should probably insert an absolutely position element at the end of the 'range.' This works differently in different browsers, so your best bet might be to sniff.

And since you asked: this is how the new york times does it in their 'altClickToSearch.js' file:

function insertButton() {

selectionButton = new Element(
        'span', {
          'className':'nytd_selection_button',
          'id':'nytd_selection_button',
          'title':'Lookup Word',
          'style': 'margin:-20px 0 0 -20px; position:absolute; background:url(http://graphics8.nytimes.com/images/global/word_reference/ref_bubble.png);width:25px;height:29px;cursor:pointer;_background-image: none;filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src="http://graphics8.nytimes.com/images/global/word_reference/ref_bubble.png", sizingMethod="image");'
        }
    )

if (Prototype.Browser.IE) {
  var tmp = new Element('div');
  tmp.appendChild(selectionButton);
  newRange = selection.duplicate();
  newRange.setEndPoint( "StartToEnd", selection);
  newRange.pasteHTML(tmp.innerHTML);
  selectionButton = $('nytd_selection_button');
}
else {
  var range = selection.getRangeAt(0);
  newRange = document.createRange();
  newRange.setStart(selection.focusNode, range.endOffset);
  newRange.insertNode(selectionButton);
}
}

I use getBoundingClientRect() when I need the content to remain undisturbed, while placing additional content near it.

    var r=window.getSelection().getRangeAt(0).getBoundingClientRect();
    var relative=document.body.parentNode.getBoundingClientRect();
    ele.style.top =(r.bottom -relative.top)+'px';//this will place ele below the selection
    ele.style.right=-(r.right-relative.right)+'px';//this will align the right edges together

this works in Chrome, but IE likes to give weird things, so here's a cross-browser solution: (Tested in Chrome and IE, probably works elsewhere)

https://jsfiddle.net/joktrpkz/7/