CellTree bugfix: disappears when clicking in the widget but outside tree nodes

Contributed by tbroyer
Fixes issue 6359
Review at https://gwt-code-reviews.appspot.com/1827803/

Review by: cromwellian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11265 8db76d5a-ed1c-0410-87a9-c151d255dfc7

user/src/com/google/gwt/user/cellview/client/CellTree.java

diff --git a/user/src/com/google/gwt/user/cellview/client/CellTree.java b/user/src/com/google/gwt/user/cellview/client/CellTree.java
index 7b12a0a..4847ee6 100644
--- a/user/src/com/google/gwt/user/cellview/client/CellTree.java
+++ b/user/src/com/google/gwt/user/cellview/client/CellTree.java
@@ -740,7 +740,7 @@
     if (nodeView != null) {
       if (isMouseDown) {
         Element showMoreElem = nodeView.getShowMoreElement();
-        if (nodeView.getImageElement().isOrHasChild(target)) {
+        if (!nodeView.isRootNode() && nodeView.getImageElement().isOrHasChild(target)) {
           // Open the node when the open image is clicked.
           nodeView.setOpen(!nodeView.isOpen(), true);
           return;