Tab dragging as implemented as a mus API.

components/mus/public/interfaces/window_tree.mojom

Index: components/mus/public/interfaces/window_tree.mojom
diff --git a/components/mus/public/interfaces/window_tree.mojom b/components/mus/public/interfaces/window_tree.mojom
index 878b93f556280c41e2517690f222324ec3e4abd1..d1692baddb89f50a5e550589e5fb7bf627e9c57d 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -229,6 +229,16 @@ interface WindowTree {
   // 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
@@ -358,6 +368,9 @@ interface WindowTreeClient {
   // 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);