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

ash/wm/window_util.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/wm/window_util.h"
 
 #include "ash/wm/activation_controller.h"
 #include "ui/aura/client/activation_client.h"
 #include "ui/aura/client/aura_constants.h"
 #include "ui/aura/root_window.h"
 #include "ui/aura/window.h"
 #include "ui/base/ui_base_types.h"
 #include "ui/gfx/screen.h"
 
 namespace ash {
 
+namespace {
+
+const char kOpenWindowSplitKey[] = "OpenWindowSplit";
+
+}  // namespace
+
 void ActivateWindow(aura::Window* window) {
   aura::client::GetActivationClient()->ActivateWindow(window);
 }
 
 void DeactivateWindow(aura::Window* window) {
   aura::client::GetActivationClient()->DeactivateWindow(window);
 }
 
 bool IsActiveWindow(aura::Window* window) {
   return GetActiveWindow() == window;
@@ skipping 22 matching lines @@
 bool HasFullscreenWindow(const WindowSet& windows) {
   for (WindowSet::const_iterator i = windows.begin(); i != windows.end(); ++i) {
     if ((*i)->GetIntProperty(aura::client::kShowStateKey)
         == ui::SHOW_STATE_FULLSCREEN) {
       return true;
     }
   }
   return false;
 }
 
+void SetOpenWindowSplit(aura::Window* window, bool value) {
+  window->SetIntProperty(kOpenWindowSplitKey, value ? 1 : 0);
+}
+
+bool GetOpenWindowSplit(aura::Window* window) {
+  return window->GetIntProperty(kOpenWindowSplitKey) == 1;
+}
+
 }  // namespace window_util
 }  // namespace ash