Index: /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineWindowFullscreen.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineWindowFullscreen.cpp	(revision 73520)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/runtime/fullscreen/UIMachineWindowFullscreen.cpp	(revision 73521)
@@ -353,4 +353,7 @@
     /* Resize window to the appropriate size on ML and next if it's screen has no own user-space: */
     else if (!pFullscreenLogic->screensHaveSeparateSpaces() && m_uScreenId != 0)
+        resize(workingArea.size());
+    /* Resize the window if we are already in the full screen mode. This covers cases like host-resolution changes while in full screen mode: */
+    else if(darwinIsInFullscreenMode(this))
         resize(workingArea.size());
     else