Fix zombie windows in Mac OS when switching to/from fullscreen

src/SFML/Window/OSX/SFWindow.h

@@ -57,6 +57,18 @@
 ////////////////////////////////////////////////////////////
 -(BOOL)canBecomeKeyWindow;
 
+////////////////////////////////////////////////////////////
+/// \brief Allow fullscreen windows to become the main window
+///
+/// By default, fullscreen windows (using NSBorderlessWindowMask)
+/// cannot become the main window, so this method is changed to
+/// always return YES.
+///
+/// \return YES
+///
+////////////////////////////////////////////////////////////
+-(BOOL)canBecomeMainWindow;
+
 ////////////////////////////////////////////////////////////
 /// \brief Prevent system alert
 ///

src/SFML/Window/OSX/SFWindow.m

@@ -45,6 +45,13 @@
 }
 
 
+////////////////////////////////////////////////////////
+-(BOOL)canBecomeMainWindow
+{
+    return YES;
+}
+
+
 ////////////////////////////////////////////////////////
 -(void)keyDown:(NSEvent*)theEvent
 {

src/SFML/Window/OSX/WindowImplCocoa.mm

@@ -154,7 +154,11 @@ WindowImplCocoa::~WindowImplCocoa()
     // Put the next window in front, if any.
     NSArray* windows = [NSApp orderedWindows];
     if ([windows count] > 0)
-        [[windows objectAtIndex:0] makeKeyAndOrderFront:nil];
+    {
+        NSWindow *nextWindow = [windows objectAtIndex:0];
+        if ([nextWindow isVisible])
+            [nextWindow makeKeyAndOrderFront:nil];
+    }
 
     drainThreadPool(); // Make sure everything was freed
     // This solve some issue when sf::Window::Create is called for the