Tab dragging as implemented as a mus API.

components/mus/public/interfaces/window_manager.mojom

Index: components/mus/public/interfaces/window_manager.mojom
diff --git a/components/mus/public/interfaces/window_manager.mojom b/components/mus/public/interfaces/window_manager.mojom
index ed0ffe172a1023bdfbbf6e7dd0af7c276247af1f..e96e5a948c66497ea101d679e5a7b7b103fdea2f 100644
--- a/components/mus/public/interfaces/window_manager.mojom
+++ b/components/mus/public/interfaces/window_manager.mojom
@@ -83,6 +83,16 @@ interface WindowManager {
   // 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);
 };
@@ -131,4 +141,8 @@ interface WindowManagerClient {
   // 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() ?

 };