Reworks the workspace code. Here's the new heuristics:

ash/wm/workspace/workspace_event_filter.h

Index: ash/wm/workspace/workspace_event_filter.h
diff --git a/ash/wm/workspace/workspace_event_filter.h b/ash/wm/workspace/workspace_event_filter.h
index d6765fa38b54c1da99bf902831024b56a83a23b2..6224c1099b5b9df982af2383329f5a2772c475d0 100644
--- a/ash/wm/workspace/workspace_event_filter.h
+++ b/ash/wm/workspace/workspace_event_filter.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include "ash/wm/toplevel_window_event_filter.h"
+#include "ui/aura/window_observer.h"
 
 namespace aura {
 class MouseEvent;
@@ -16,7 +17,8 @@ class Window;
 namespace ash {
 namespace internal {
 
-class WorkspaceEventFilter : public ToplevelWindowEventFilter {
+class WorkspaceEventFilter : public ToplevelWindowEventFilter,
+                             public aura::WindowObserver {
  public:
   explicit WorkspaceEventFilter(aura::Window* owner);
   virtual ~WorkspaceEventFilter();
@@ -25,6 +27,9 @@ class WorkspaceEventFilter : public ToplevelWindowEventFilter {
   virtual bool PreHandleMouseEvent(aura::Window* target,
                                    aura::MouseEvent* event) OVERRIDE;
 
+  // Overriden from WindowObserver:
+  virtual void OnWindowDestroyed(aura::Window* window) OVERRIDE;
+
  private:
   enum DragState {
     DRAG_NONE,