NativeMessageBox: Fix result codes on Linux

modules/juce_gui_basics/native/juce_linux_NativeMessageBox.cpp

@@ -28,7 +28,42 @@ namespace juce
 
 std::unique_ptr<ScopedMessageBoxInterface> ScopedMessageBoxInterface::create (const MessageBoxOptions& options)
 {
-    return createAlertWindowImpl (options);
+    // On Linux, we re-use the AlertWindow rather than using a platform-specific dialog.
+    // For consistency with the NativeMessageBox on other platforms, the result code must
+    // match the button index, hence this adapter.
+    class MessageBox : public ScopedMessageBoxInterface
+    {
+    public:
+        explicit MessageBox (const MessageBoxOptions& options)
+            : inner (createAlertWindowImpl (options)),
+              numButtons (options.getNumButtons()) {}
+
+        void runAsync (std::function<void (int)> fn) override
+        {
+            inner->runAsync ([fn, n = numButtons] (int result)
+                             {
+                                 fn (map (result, n));
+                             });
+        }
+
+        int runSync() override
+        {
+            return map (inner->runSync(), numButtons);
+        }
+
+        void close() override
+        {
+            inner->close();
+        }
+
+    private:
+        static int map (int button, int numButtons) { return (button + numButtons - 1) % numButtons; }
+
+        std::unique_ptr<ScopedMessageBoxInterface> inner;
+        int numButtons = 0;
+    };
+
+    return std::make_unique<MessageBox> (options);
 }
 
 } // namespace juce