Bug 606827: Released and collapsing drag regions fade into... somewhere [r+a=dolske]

--HG--
extra : rebase_source : b225624a3e2352ce72b4ce5d90f3f53da141a8ac

browser/base/content/tabview/ui.js

@@ -1023,11 +1023,12 @@ let UI = {
     }
 
     function collapse() {
+      let center = phantom.bounds().center();
       phantom.animate({
         width: 0,
         height: 0,
-        top: phantom.position().x + phantom.height()/2,
+        top: center.y,
-        left: phantom.position().y + phantom.width()/2
+        left: center.x
       }, {
         duration: 300,
         complete: function() {