Tab dragging as implemented as a mus API.

components/mus/public/interfaces/window_manager.mojom

 // Copyright 2015 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 mus.mojom;
 
 import "components/mus/public/interfaces/cursor.mojom";
 import "components/mus/public/interfaces/event_matcher.mojom";
 import "components/mus/public/interfaces/window_manager_constants.mojom";
 import "components/mus/public/interfaces/window_tree_constants.mojom";
@@ skipping 65 matching lines @@
   WmCreateTopLevelWindow(uint32 change_id,
                          uint16 requesting_client_id,
                          map<string, array<uint8>> properties);
 
   // A WindowTreeClient is considered "janky" by Mus when it stops ACK'ing input
   // events within a reasonable timeframe. When a client enters or exits this
   // state, Mus will tell the window manager about it so that the window manager
   // can update the UI for the janky windows.
   WmClientJankinessChanged(uint16 client_id, bool janky);
 
+  // Asks the WindowManager to interactively move the window. This will return
+  // whether this completed successfully or not through the client's
+  // OnWmMoveLoopCompleted().
+  WmPerformMoveLoop(uint32 change_id, uint32 window_id,
+                    MoveLoopSource source,
+                    gfx.mojom.Point cursor_location);
+
+  // Asks the WindowManager to cancel any outstanding move loop on |window_id|.
+  WmCancelMoveLoop(uint32 window_id);
+
   // An accelerator registered via AddAccelerator() has been triggered.
   OnAccelerator(uint32 id, ui.mojom.Event event);
 };
 
 // This interface is only used as an associated interface and is associated
 // with WindowTree.
 interface WindowManagerClient {
   // Enables (or disables) child windows of |window_id| to be activated.
   AddActivationParent(uint32 window_id);
   RemoveActivationParent(uint32 window_id);
@@ skipping 28 matching lines @@
   // Sets the frame decoration constants of the display the window manager is
   // associated with.
   WmSetFrameDecorationValues(FrameDecorationValues values);
 
   // Sets the cursor that the non-client areas of the window should use.
   WmSetNonClientCursor(uint32 window_id, Cursor cursor_id);
 
   // Response from WmCreateTopLevelWindow() informing the client of the id for
   // the new window.
   OnWmCreatedTopLevelWindow(uint32 change_id, uint32 window_id);
+
+  // Response from WmPerformMoveLoop(). Returns true if the move loop was
+  // completed successfully.
+  OnWmMoveLoopCompleted(uint32 change_id, bool completed);

[sky, 2016/06/29 00:04:08]

Can't the client call WmResponse() ?

 };