Index: /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin-cocoa.mm
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin-cocoa.mm	(revision 52592)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin-cocoa.mm	(revision 52593)
@@ -184,4 +184,12 @@
         [pWindow setHasShadow:YES];
     }
+}
+
+void darwinSetWindowHasShadow(NativeNSWindowRef pWindow, bool fEnabled)
+{
+    if (fEnabled)
+        [pWindow setHasShadow :YES];
+    else
+        [pWindow setHasShadow :NO];
 }
 
Index: /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin.cpp	(revision 52592)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin.cpp	(revision 52593)
@@ -82,4 +82,9 @@
 {
     ::darwinSetShowsWindowTransparentImpl(::darwinToNativeWindow(pWidget), fEnabled);
+}
+
+void darwinSetWindowHasShadow(QWidget *pWidget, bool fEnabled)
+{
+    ::darwinSetWindowHasShadow(::darwinToNativeWindow(pWidget), fEnabled);
 }
 
Index: /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin.h
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin.h	(revision 52592)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/platform/darwin/VBoxUtils-darwin.h	(revision 52593)
@@ -62,4 +62,5 @@
 void darwinLabelWindow(NativeNSWindowRef pWindow, NativeNSImageRef pImage, bool fCenter);
 void darwinSetShowsWindowTransparentImpl(NativeNSWindowRef pWindow, bool fEnabled);
+void darwinSetWindowHasShadow(NativeNSWindowRef pWindow, bool fEnabled);
 void darwinSetMouseCoalescingEnabled(bool fEnabled);
 
@@ -229,4 +230,5 @@
 void darwinSetHidesAllTitleButtons(QWidget *pWidget);
 void darwinSetShowsWindowTransparent(QWidget *pWidget, bool fEnabled);
+void darwinSetWindowHasShadow(QWidget *pWidget, bool fEnabled);
 void darwinSetDockIconMenu(QMenu *pMenu);
 void darwinDisableIconsInMenus(void);
Index: /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UISlidingToolBar.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UISlidingToolBar.cpp	(revision 52592)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UISlidingToolBar.cpp	(revision 52593)
@@ -264,4 +264,26 @@
 }
 
+bool UISlidingToolBar::event(QEvent *pEvent)
+{
+    /* Depending on event-type: */
+    switch (pEvent->type())
+    {
+        case QEvent::Resize:
+        case QEvent::WindowActivate:
+        {
+            /* By some strange reason
+             * cocoa resets NSWindow::setHasShadow option
+             * for frameless windows on every window resize/activation.
+             * So we have to make sure window still has no shadows. */
+            darwinSetWindowHasShadow(this, false);
+            break;
+        }
+        default:
+            break;
+    }
+    /* Call to base-class: */
+    return QWidget::event(pEvent);
+}
+
 void UISlidingToolBar::setWidgetGeometry(const QRect &rect)
 {
Index: /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UISlidingToolBar.h
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UISlidingToolBar.h	(revision 52592)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UISlidingToolBar.h	(revision 52593)
@@ -93,4 +93,6 @@
     /** Close event handler. */
     virtual void closeEvent(QCloseEvent *pEvent);
+    /** Common event handler. */
+    virtual bool event(QEvent *pEvent);
 
     /** Defines mdi-sub-window geometry. */