Index: /trunk/src/VBox/Frontends/VirtualBox/include/VBoxFBOverlay.h
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/include/VBoxFBOverlay.h	(revision 23656)
+++ /trunk/src/VBox/Frontends/VirtualBox/include/VBoxFBOverlay.h	(revision 23657)
@@ -91,6 +91,20 @@
 
 #define VBOXQGLLOG_METHODTIME(_m) VBoxVHWADbgTimeCounter _dbgTimeCounter(_m)
+
+#define VBOXQG_CHECKCONTEXT() \
+        { \
+            const GLubyte * str; \
+            VBOXQGL_CHECKERR(   \
+                    str = glGetString(GL_VERSION); \
+            ); \
+            Assert(str); \
+            if(str) \
+            { \
+                Assert(str[0]); \
+            } \
+        }
 #else
 #define VBOXQGLLOG_METHODTIME(_m)
+#define VBOXQG_CHECKCONTEXT() do{}while(0)
 #endif
 
Index: /trunk/src/VBox/Frontends/VirtualBox/src/VBoxFBOverlay.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/VBoxFBOverlay.cpp	(revision 23656)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/VBoxFBOverlay.cpp	(revision 23657)
@@ -5573,4 +5573,7 @@
 
         vboxShowOverlay(true);
+
+        /* workaround for linux ATI issue: need to update gl viewport after widget becomes visible */
+        mpOverlayWidget->vboxDoUpdateViewport(overVp);
     }
 }