Fix dock mode with 2+ external displays.

ui/display/chromeos/update_display_configuration_task.cc

 // Copyright 2014 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 "ui/display/chromeos/update_display_configuration_task.h"
 
 #include "ui/display/chromeos/configure_displays_task.h"
 #include "ui/display/chromeos/display_layout_manager.h"
 #include "ui/display/chromeos/display_util.h"
 #include "ui/display/types/display_snapshot.h"
@@ skipping 163 matching lines @@
     return true;
 
   if (new_display_state_ != layout_manager_->GetDisplayState())
     return true;
 
   return false;
 }
 
 MultipleDisplayState UpdateDisplayConfigurationTask::ChooseDisplayState()
     const {
+  int num_displays = cached_displays_.size();
   int num_on_displays =
-      GetDisplayPower(cached_displays_, new_power_state_, NULL);
-  switch (cached_displays_.size()) {
-    case 0:
-      return MULTIPLE_DISPLAY_STATE_HEADLESS;
-    case 1:
-      return MULTIPLE_DISPLAY_STATE_SINGLE;
-    default: {
-      if (num_on_displays == 1) {
-        // If only one display is currently turned on, return the "single"
-        // state so that its native mode will be used.
-        return MULTIPLE_DISPLAY_STATE_SINGLE;
-      }
-      if (num_on_displays >= 3) {
-        return MULTIPLE_DISPLAY_STATE_MULTI_EXTENDED;
-      } else if (cached_displays_.size() == 2) {
-        if (!layout_manager_->GetStateController())
-          return MULTIPLE_DISPLAY_STATE_DUAL_EXTENDED;
-        // With either both displays on or both displays off, use one of the
-        // dual modes.
-        return layout_manager_->GetStateController()->GetStateForDisplayIds(
-            cached_displays_);
-      }
-      NOTREACHED();
-    }
+      GetDisplayPower(cached_displays_, new_power_state_, nullptr);
+
+  if (num_displays == 0)
+    return MULTIPLE_DISPLAY_STATE_HEADLESS;
+
+  if (num_displays == 1 || num_on_displays == 1) {
+    // If only one display is currently turned on, return the "single" state
+    // so that its native mode will be used.
+    return MULTIPLE_DISPLAY_STATE_SINGLE;
   }
+
+  if (num_displays == 2 || num_on_displays == 2) {
+    // Try to use the saved configuration; otherwise, default to extended.
+    DisplayConfigurator::StateController* state_controller =
+        layout_manager_->GetStateController();
+
+    if (!state_controller)
+      return MULTIPLE_DISPLAY_STATE_DUAL_EXTENDED;
+    return state_controller->GetStateForDisplayIds(cached_displays_);
+  }
+
+  if (num_on_displays >= 3) {
+    // 3+ displays are always extended
+    return MULTIPLE_DISPLAY_STATE_MULTI_EXTENDED;
+  }
+
   return MULTIPLE_DISPLAY_STATE_INVALID;
 }
 
 }  // namespace ui