Index: /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineLogicFullscreen.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineLogicFullscreen.cpp	(revision 50520)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineLogicFullscreen.cpp	(revision 50521)
@@ -158,7 +158,8 @@
         /* Request 'normal' (window) visual-state: */
         uisession()->setRequestedVisualState(UIVisualStateType_Normal);
-        /* Toggle native fullscreen mode for each window: */
-        foreach (UIMachineWindow *pMachineWindow, machineWindows())
-            darwinToggleFullscreenMode(pMachineWindow);
+        /* Exit native fullscreen mode for each window: */
+        foreach (UIMachineWindow *pMachineWindow, machineWindows())
+            if (darwinIsInFullscreenMode(pMachineWindow))
+                darwinToggleFullscreenMode(pMachineWindow);
     }
 }
@@ -174,7 +175,8 @@
         /* Request 'seamless' visual-state: */
         uisession()->setRequestedVisualState(UIVisualStateType_Seamless);
-        /* Toggle native fullscreen mode for each window: */
-        foreach (UIMachineWindow *pMachineWindow, machineWindows())
-            darwinToggleFullscreenMode(pMachineWindow);
+        /* Exit native fullscreen mode for each window: */
+        foreach (UIMachineWindow *pMachineWindow, machineWindows())
+            if (darwinIsInFullscreenMode(pMachineWindow))
+                darwinToggleFullscreenMode(pMachineWindow);
     }
 }
@@ -190,7 +192,8 @@
         /* Request 'scale' visual-state: */
         uisession()->setRequestedVisualState(UIVisualStateType_Scale);
-        /* Toggle native fullscreen mode for each window: */
-        foreach (UIMachineWindow *pMachineWindow, machineWindows())
-            darwinToggleFullscreenMode(pMachineWindow);
+        /* Exit native fullscreen mode for each window: */
+        foreach (UIMachineWindow *pMachineWindow, machineWindows())
+            if (darwinIsInFullscreenMode(pMachineWindow))
+                darwinToggleFullscreenMode(pMachineWindow);
     }
 }
Index: /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineWindowFullscreen.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineWindowFullscreen.cpp	(revision 50520)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineWindowFullscreen.cpp	(revision 50521)
@@ -79,6 +79,7 @@
     AssertReturnVoid(vboxGlobal().osRelease() > MacOSXRelease_Lion);
 
-    /* Toggle native fullscreen mode: */
-    darwinToggleFullscreenMode(this);
+    /* Enter native fullscreen mode: */
+    if (!darwinIsInFullscreenMode(this))
+        darwinToggleFullscreenMode(this);
 }
 #endif /* RT_OS_DARWIN */