Why differences in the way different machines handle the same code??

Question

OK, I've posted some other threads re some difficulties in getting the same code to function identically on, not only different platforms, but even on different machines running Windows!
Here's another, and I would really appreciate an explanation:
I have a tree control. Per the user's request, I made the right button function the same way it does on MS File Explorer: When they right click, the currently selected nodes are compared to the node they clicked over. If they clicked over a selected node, then the context menu for the nodes selected is displayed. If they clicked over a different node, then the selection is changed to the new node and the context menu displayed. If they right click over the tree control, but not a node in the tree, then the selection is cleared and no context menu is displayed.
Works great on my machine.
However, on some machines, no context menu is displayed, nor is the node selected... UNLESS THEY ALREADY HAVE A NODE SELECTED. I.e., if they have no nodes in the tree selected, they cannot display the context menu by right clicking on a node, nor does the selection change to the node they right click over. It only works if they already have something selected.
My app is distributed as an executable jar file and I am distributing the JRE with it, so there are no class path issues or differences in the JDK. Basically, my problem can be paraphrased as: Why doesnt' Swing behave the same way on different machines...? At least why not the same on the eame machine.
It's driving me nuts to think that I've resolved some UI glitch only to find out that, no, it's only fixed on some machines, not others.
Thanks.

Answer 1

Well, I normally develop on OSX and then deploy to Windows. But, this last build I have switched to Windows XP. The kinds of differences I am seeing are subtle: what triggers a repaint of the UI and now, it seems, what events are getting triggered by mouse clicks.
My deployment is an executable jar file, therefore it uses the manifest in the jar, not any classpath variables. I deploy an entire JRE along with the app, and the bat file that launches the app ensures that is the JAVA_HOME I'm using.
I can only guess that it's some underlying difference in versions of the platform and OS that causes this.
Here's the entire code for this latest issue re the popups:
        private void maybeShowPopup(NodeInfoTree tree, MouseEvent e) {
            if (e.isPopupTrigger()) {
                //  They clicked the platform dependent popup trigger...
                int x = e.getX();
                int y = e.getY();
                TreePath[] selectedPaths = tree.getSelectionPaths();
                if (selectedPaths == null) {
                    selectedPaths = new TreePath[0];
                TreePath clickedPath = tree.getPathForLocation(x,y);
                if (clickedPath == null) {
                    //  they right clicked, but not on a node, so clear the selection and return
                    tree.clearSelection();
                    return;
                } else {
                    // They clicked on a path, so let's see if it is a selected
                    // node.  Search the selected array of paths, short circuiting
                    // out if it is found...
                    boolean foundInSelection = false;
                    for (int i=0; (!foundInSelection && (i < selectedPaths.length)); i++) {
                        foundInSelection = clickedPath.equals(selectedPaths);
if (!foundInSelection) {
// They right clicked elsewhere than the selection, so
// reset the selection to the new path...
tree.setSelectionPath(clickedPath);
final TreePath menuPath;
selectedPaths = tree.getSelectionPaths();
if (selectedPaths == null) {
return;
} else {
menuPath = selectedPaths[selectedPaths.length-1];
// Since they did click on a node, now we need to determine
// what popup menu should be displayed...
tree.scrollPathToVisible(menuPath);
NodeInfo ni = (NodeInfo) menuPath.getLastPathComponent();
// If the last path component is null, then they don't have
// anything selected, so we do nothing
if (ni == null) return;
// See if the Node is overriding our default popup menu...
com.harcourt.applications.tgen.browser.nodes.NodeMenu nodeMenu = ni.getMenu();
nodeMenu.activate(tree.getView(),ni);
JPopupMenu popup = (JPopupMenu) nodeMenu;
if (popup != null) {
popup.show(e.getComponent(),
e.getX(), e.getY());
The nodeMenu.activate call is just used to check whether or not certain menu options need to be enabled or disabled before displaying the menu. My theory right now is that, on some of these machines (all windows), a right click of the mouse is not generating a mouse event unless they have already something selected. I don't think it's a focus issue, because if they left click to select a node, and then ctrl-left click to de-select it, the right button then no longer generates a popup menu. They must select a node, then right click. Maybe there's no mouse event generated, or maybe this line is failing on some machines?:
TreePath clickedPath = tree.getPathForLocation(x,y);