Ignore fullscreen windows which are behind other windows for fullscreen mode.

ash/root_window_controller.h

Index: ash/root_window_controller.h
diff --git a/ash/root_window_controller.h b/ash/root_window_controller.h
index 076bfe6a4f82e080f4b229131f53cd9c45148102..478957d2b0807fbb5c3a94df61ffe06c0b7dbd41 100644
--- a/ash/root_window_controller.h
+++ b/ash/root_window_controller.h
@@ -221,8 +221,8 @@ class ASH_EXPORT RootWindowController : public ShellObserver {
   // Initialize touch HUDs if necessary.
   void InitTouchHuds();
 
-  // Returns the window, if any, which is in fullscreen mode. If multiple
-  // windows are in fullscreen state, the topmost one is preferred.
+  // Returns the topmost window or one of its transient parents, if any of them
+  // are in fullscreen mode.
   const aura::Window* GetTopmostFullscreenWindow() const;
   aura::Window* GetTopmostFullscreenWindow();