aura-mus: Implement stacking in DesktopWindowTreeHostMus.

services/ui/ws/window_tree.cc

Index: services/ui/ws/window_tree.cc
diff --git a/services/ui/ws/window_tree.cc b/services/ui/ws/window_tree.cc
index e424cf317f94becfbcf48d41513cabe789e1dcec..28cea21d49d0829771c02d16c370534b64fdfb24 100644
--- a/services/ui/ws/window_tree.cc
+++ b/services/ui/ws/window_tree.cc
@@ -1605,6 +1605,44 @@ void WindowTree::DeactivateWindow(Id window_id) {
       wm_tree->ClientWindowIdForWindow(window).id);
 }
 
+void WindowTree::StackAtTop(uint32_t change_id, Id window_id) {
+  ServerWindow* window = GetWindowByClientId(ClientWindowId(window_id));
+  if (!window) {
+    DVLOG(1) << "StackAtTop failed (invalid id)";
+    client()->OnChangeCompleted(change_id, false);
+    return;
+  }
+
+  if (!access_policy_->CanStackAtTop(window)) {
+    DVLOG(1) << "StackAtTop failed (access denied)";
+    client()->OnChangeCompleted(change_id, false);
+    return;
+  }
+
+  ServerWindow* parent = window->parent();
+  if (!parent) {
+    DVLOG(1) << "StackAtTop failed (window unparented)";
+    client()->OnChangeCompleted(change_id, false);
+    return;
+  }
+
+  DCHECK(!parent->children().empty());
+  if (parent->children().back() == window) {
+    // Ignore this call; the client didn't know they were already at the top.
+    DVLOG(3) << "StackAtTop ignored (already at top)";
+    client()->OnChangeCompleted(change_id, true);
+    return;
+  }
+
+  ServerWindow* relative_window = parent->children().back();
+  Operation op(this, window_server_, OperationType::REORDER_WINDOW);
+  window->Reorder(relative_window, mojom::OrderDirection::ABOVE);
+  window_server_->ProcessWindowReorder(window, relative_window,
+                                       mojom::OrderDirection::ABOVE);
+
+  client()->OnChangeCompleted(change_id, true);
+}
+
 void WindowTree::GetWindowManagerClient(
     mojo::AssociatedInterfaceRequest<mojom::WindowManagerClient> internal) {
   if (!access_policy_->CanSetWindowManager() || !window_manager_internal_ ||