feature: when a WindowTree is removed, then we add its contents to any other tree that we can find

src/miracle_window_management_policy.cpp

@@ -273,7 +273,10 @@ void MiracleWindowManagementPolicy::advise_output_delete(miral::Output const& ou
                 if (tree_list.empty())
                     active_tree = nullptr;
                 else
+                {
                     active_tree = tree_list[0];
+                    active_tree->tree.add_tree(it->tree);
+                }
             }
             break;
        }

src/node.h

@@ -89,7 +89,7 @@ public:
     bool is_window() const { return state == NodeState::window; }
     bool is_lane() const { return state == NodeState::lane; }
     NodeLayoutDirection get_direction() const { return direction; }
-    miral::Window const& get_window() const { return window; }
+    miral::Window& get_window() { return window; }
     std::shared_ptr<Node> get_parent() const { return parent; }
     std::vector<std::shared_ptr<Node>> const& get_sub_nodes() const { return sub_nodes; }
 

src/window_tree.cpp

@@ -619,4 +619,35 @@ bool WindowTree::constrain(miral::WindowInfo &window_info)
 
     node->get_parent()->constrain();
     return true;
+}
+
+void WindowTree::add_tree(WindowTree& other_tree)
+{
+    other_tree.foreach_node([&](auto node)
+    {
+        if (node->is_window())
+        {
+            auto new_node_position = root_lane->create_new_node_position();
+            node->set_logical_area(new_node_position);
+            root_lane->add_window(node->get_window());
+        }
+    });
+}
+
+namespace
+{
+void foreach_node_internal(std::function<void(std::shared_ptr<Node>)> f, std::shared_ptr<Node> parent)
+{
+    f(parent);
+    if (parent->is_window())
+        return;
+
+    for (auto node : parent->get_sub_nodes())
+        foreach_node_internal(f, node);
+}
+}
+
+void WindowTree::foreach_node(std::function<void(std::shared_ptr<Node>)> f)
+{
+    foreach_node_internal(f, root_lane);
 }

src/window_tree.h

@@ -94,6 +94,10 @@ public:
     /// Constrains the window to its tile if it is in this tree.
     bool constrain(miral::WindowInfo& window_info);
 
+    void add_tree(WindowTree&);
+
+    void foreach_node(std::function<void(std::shared_ptr<Node>)>);
+
 private:
     miral::WindowManagerTools tools;
     WindowTreeOptions options;