Tab dragging as implemented as a mus API.

components/mus/public/interfaces/window_tree.mojom

 // 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.
 
 module mus.mojom;
 
 import "cc/ipc/surface_id.mojom";
 import "components/mus/public/interfaces/cursor.mojom";
 import "components/mus/public/interfaces/event_matcher.mojom";
 import "components/mus/public/interfaces/mus_constants.mojom";
@@ skipping 211 matching lines @@
   // See documentation for WindowTreeClient::OnWindowInputEvent().
   OnWindowInputEventAck(uint32 event_id, EventResult result);
 
   // See description of WindowManager for details.
   GetWindowManagerClient(associated WindowManagerClient& internal);
 
   // Returns a shared memory segment that contains two 16-bit ints packed into a
   // single Atomic32, which represent the current location of the mouse cursor
   // where the location is (x << 16) | y.
   GetCursorLocationMemory() => (handle<shared_buffer> cursor_buffer);
+
+  // Tells the window manager to start moving the window. OnChangeCompleted is
+  // called on whether the move was canceled. Because there's a delay between
+  // when a client sends this message and when the window manager starts acting

[sky, 2016/06/22 23:47:56]

There is a delay, but isn't this sent before the client has ack'd the current
event so that mus should know the cursor position it sent to the client and can
pass that to the windowmanager?

[Elliot Glaysher, 2016/06/24 17:41:55]

Experimentally, the delay is actually pretty big and can lead to the cursor
outside the tab. (I don't entirely understand why they aren't equivalent
though.)

[sky, 2016/06/24 19:27:00]

It's worth understanding why this happens.

+  // on it, pass the cursor location at the start of the move.
+  PerformWindowMove(uint32 change_id, uint32 window_id, gfx.mojom.Point cursor);
+
+  // Tells the window manager to cancel any in progress window move started with
+  // StartWindowMove() and to revert the window bounds to how they were.
+  CancelWindowMove(uint32 window_id);
 };
 
 // Changes to windows are not sent to the connection that originated the
 // change. For example, if connection 1 changes the bounds of a window by
 // calling SetWindowBounds(), connection 1 does not receive
 // OnWindowBoundsChanged().
 interface WindowTreeClient {
   // Invoked when the client application has been embedded at |root|.
   // See Embed() on WindowTree for more details. |tree| will be a handle back to
   // the window manager service, unless the connection is to the root connection
@@ skipping 109 matching lines @@
   // is 0. As with other functions this is only called if the client did not
   // initiate the change.
   OnWindowFocused(uint32 focused_window_id);
 
   OnWindowPredefinedCursorChanged(uint32 window_id, Cursor cursor_id);
 
   // A change initiated from the client has completed. See description of
   // change ids for details.
   OnChangeCompleted(uint32 change_id, bool success);
 
+  // Called in response to PerformWindowMove().
+  OnMoveLoopCompleted(uint32 change_id, bool success);

[sky, 2016/06/22 23:47:57]

Can't you use OnChangeCompleted() for this?

[Elliot Glaysher, 2016/06/24 17:41:55]

OnMoveLoopCompleted() does additional things like calling the closure (which
previously was directly from a run loop, and now, is indirectly from a run loop
passed in from mus.)

[sky, 2016/06/24 19:27:00]

Isn't that an implementation detail though? By that I mean you could still call
OnChangeCompleted and WindowTreeClient can know the change corresponds to a move
as it supplied the change_id.

+
   // The WindowManager is requesting the specified window to close. If the
   // client allows the change it should delete the window.
   RequestClose(uint32 window_id);
 
   // See description of WindowManager for details.
   GetWindowManager(associated WindowManager& internal);
 };
 
 // Mus provides this interface as a way for clients to connect and obtain a
 // WindowTree handle with a supplied WindowTreeClient handle. The
 // WindowTreeClient has no roots, use NewTopLevelWindow() to create one.
 interface WindowTreeFactory {
   CreateWindowTree(WindowTree& tree_request, WindowTreeClient client);
 };