Primary display change notifications.

services/ui/public/interfaces/display_manager.mojom

 // Copyright 2016 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.
 
 module ui.mojom;
 
 import "services/ui/public/interfaces/window_manager_constants.mojom";
 
 interface DisplayManagerObserver {
-  // Sent when the observer is added. Gives the initial state.
-  OnDisplays(array<WsDisplay> displays);
+  // Sent when the observer is added. |displays| contains all known displays.
+  // |internal_display_id| will be the display id of an existing display or 

[sky, 2016/10/28 17:08:47]

It's not clear what you mean by internal in this context?

[kylechar, 2016/10/28 19:40:37]

Done.

+  // kInvalidDisplayID if there is no internal display id.
+  OnDisplays(array<WsDisplay> displays,
+             int64 primary_display_id,
+             int64 internal_display_id);
 
   // The specified set of displays has changed in some way. This is used both
   // when a new display is added, or when a property of a display changes.
   // |displays| does not necessarily contain the complete set of displays,
   // only those that changed.
   OnDisplaysChanged(array<WsDisplay> displays);
 
   // An existing display was removed.
-  OnDisplayRemoved(int64 id);
+  OnDisplayRemoved(int64 display_id);
+
+  // The primary display has changed. If the last display is about to be removed
+  // then |primary_display_id| will be kInvalidDisplayID.
+  OnPrimaryDisplayChanged(int64 primary_display_id);

[sky, 2016/10/28 17:08:47]

Similar comment to PlatformScreenDelegate.

[kylechar, 2016/10/28 19:40:36]

Done.

 };
 
 interface DisplayManager {
   AddObserver(DisplayManagerObserver observer);
 };