Index: user/src/com/google/gwt/user/client/ui/PopupPanel.java
===================================================================
--- user/src/com/google/gwt/user/client/ui/PopupPanel.java	(revision 5877)
+++ user/src/com/google/gwt/user/client/ui/PopupPanel.java	(working copy)
@@ -389,7 +389,8 @@
 
     int left = (Window.getClientWidth() - getOffsetWidth()) >> 1;
     int top = (Window.getClientHeight() - getOffsetHeight()) >> 1;
-    setPopupPosition(Window.getScrollLeft() + left, Window.getScrollTop() + top);
+    setPopupPosition(Math.max(Window.getScrollLeft() + left, 0), Math.max(
+        Window.getScrollTop() + top, 0));
 
     if (!initiallyShowing) {
       setAnimationEnabled(initiallyAnimated);