diff -r 000000000000 -r 6474c204b198 browser/components/tabview/test/browser_tabview_bug580412.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/tabview/test/browser_tabview_bug580412.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,80 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function test() {
+  waitForExplicitFinish();
+  showTabView(onTabViewShown);
+}
+
+function onTabViewShown() {
+  let contentWindow = TabView.getContentWindow();
+  let [originalTab] = gBrowser.visibleTabs;
+
+  ok(TabView.isVisible(), "Tab View is visible");
+  is(contentWindow.GroupItems.groupItems.length, 1, "There is only one group");
+  let currentActiveGroup = contentWindow.GroupItems.getActiveGroupItem();
+
+  let endGame = function() {
+    ok(TabView.isVisible(), "TabView is shown");
+    gBrowser.selectedTab = originalTab;
+
+    hideTabView(function () {
+      ok(!TabView.isVisible(), "TabView is hidden");
+      finish();
+    });
+  }
+
+  // we need to stop the setBounds() css animation or else the test will
+  // fail in single-mode because the group is newly created "ontabshown".
+  let $container = contentWindow.iQ(currentActiveGroup.container);
+  $container.css("transition-property", "none");
+
+  currentActiveGroup.setPosition(40, 40, true);
+  currentActiveGroup.arrange({animate: false});
+
+  // move down 20 so we're far enough away from the top.
+  checkSnap(currentActiveGroup, 0, 20, contentWindow, function(snapped){
+    is(currentActiveGroup.getBounds().top, 60, "group.top is 60px");
+    ok(!snapped,"Move away from the edge");
+
+    // Just pick it up and drop it.
+    checkSnap(currentActiveGroup, 0, 0, contentWindow, function(snapped){
+      is(currentActiveGroup.getBounds().top, 60, "group.top is 60px");
+      ok(!snapped,"Just pick it up and drop it");
+
+      checkSnap(currentActiveGroup, 0, 1, contentWindow, function(snapped){
+        is(currentActiveGroup.getBounds().top, 60, "group.top is 60px");
+        ok(snapped,"Drag one pixel: should snap");
+
+        checkSnap(currentActiveGroup, 0, 5, contentWindow, function(snapped){
+          is(currentActiveGroup.getBounds().top, 65, "group.top is 65px");
+          ok(!snapped,"Moving five pixels: shouldn't snap");
+          endGame();
+        });
+      });
+    });
+  });
+}
+
+function simulateDragDrop(item, offsetX, offsetY, contentWindow) {
+  let target = item.container;
+
+  EventUtils.synthesizeMouse(target, 1, 1, {type: "mousedown"}, contentWindow);
+  EventUtils.synthesizeMouse(target, 1 + offsetX, 1 + offsetY, {type: "mousemove"}, contentWindow);
+  EventUtils.synthesizeMouse(target, 1, 1, {type: "mouseup"}, contentWindow);
+}
+
+function checkSnap(item, offsetX, offsetY, contentWindow, callback) {
+  let firstTop = item.getBounds().top;
+  let firstLeft = item.getBounds().left;
+
+  simulateDragDrop(item, offsetX, offsetY, contentWindow);
+
+  let snapped = false;
+  if (item.getBounds().top != firstTop + offsetY)
+    snapped = true;
+  if (item.getBounds().left != firstLeft + offsetX)
+    snapped = true;
+
+  callback(snapped);
+}