editor: dock new Hierarchy windows with the existing one (#258)

Extend the #258 new-window placement to scene Hierarchy windows: when
Create Scene opens a new "Scene Hierarchy" window, dock (tab) it with an
existing scene-hierarchy window instead of leaving it floating at ImGui's
default cascade position; when none is docked, float it centered at 25%
width x 50% height of the viewport.

Item_tree_window is also used for the developer-only "Tools Library"
tree, so match on an explicit is_scene_hierarchy() marker (set in
Scene_root::make_browser_window) rather than the concrete type. Placement
is applied in Scene_commands::create_new_scene, alongside the viewport
placement; the startup default-scene browser and the scene-load path
(which restores its own saved per-scene layout) are left untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tksuoran

src/editor/scene/scene_commands.cpp

@@ -568,6 +568,9 @@ auto Scene_commands::create_new_scene() -> std::shared_ptr<Scene_root>
         *m_context.app_settings
     );
     browser_window->show_window();
+    // Dock (tab) the new scene's Hierarchy window with the existing one instead
+    // of leaving it floating at ImGui's default cascade position (#258).
+    apply_hierarchy_window_placement(*m_context.imgui_windows, *browser_window);
 
     // A viewport window looking through the new camera. create_viewport_scene_view
     // is called directly (rather than open_new_viewport_scene_view) so the new

src/editor/scene/scene_root.cpp

@@ -301,6 +301,7 @@ auto Scene_root::make_browser_window(
         fmt::format("Scene Hierarchy [{}]", ++s_browser_window_count),
         ""
     );
+    m_node_tree_window->set_scene_hierarchy(true);
     m_node_tree_window->set_root(m_scene->get_root_node());
     // Show a selectable Scene item at the top of the Hierarchy window, with the
     // Content Library nested under it (issue #240). The scene root node's child

src/editor/windows/item_tree_window.cpp

@@ -1748,6 +1748,16 @@ void Item_tree_window::on_end()
     ImGui::PopStyleVar(2);
 }
 
+void Item_tree_window::set_scene_hierarchy(const bool value)
+{
+    m_is_scene_hierarchy = value;
+}
+
+auto Item_tree_window::is_scene_hierarchy() const -> bool
+{
+    return m_is_scene_hierarchy;
+}
+
 void Item_tree_window::hidden()
 {
     // Drop cached rows so a hidden tree does not keep deleted items alive

src/editor/windows/item_tree_window.hpp

@@ -203,11 +203,21 @@ class Item_tree_window : public erhe::imgui::Imgui_window, public Item_tree
         std::string_view             ini_label
     );
 
+    // Marks this tree window as a scene-hierarchy browser (as opposed to the
+    // generic content-library / tools tree windows built from the same class).
+    // Used by window placement (#258) to dock a new Hierarchy window with an
+    // existing one.
+    void set_scene_hierarchy(bool value);
+    [[nodiscard]] auto is_scene_hierarchy() const -> bool;
+
     // Implements Imgui_window
     void imgui   () override;
     void on_begin() override;
     void on_end  () override;
     void hidden  () override;
+
+private:
+    bool m_is_scene_hierarchy{false};
 };
 
 }

src/editor/windows/window_placement.cpp

@@ -2,6 +2,7 @@
 
 #include "geometry_graph/geometry_graph_window.hpp"
 #include "texture_graph/texture_graph_window.hpp"
+#include "windows/item_tree_window.hpp"
 #include "windows/properties.hpp"
 #include "windows/viewport_window.hpp"
 
@@ -20,6 +21,8 @@ namespace {
 constexpr float c_editor_window_ratio     = 0.66f;
 constexpr float c_properties_width_ratio  = 0.33f;
 constexpr float c_properties_height_ratio = 0.66f;
+constexpr float c_hierarchy_width_ratio   = 0.25f;
+constexpr float c_hierarchy_height_ratio  = 0.50f;
 
 auto is_editor_window(const erhe::imgui::Imgui_window* window) -> bool
 {
@@ -89,4 +92,21 @@ void apply_properties_window_placement(erhe::imgui::Imgui_windows& imgui_windows
     new_window.set_initial_placement(target, c_properties_width_ratio, c_properties_height_ratio);
 }
 
+void apply_hierarchy_window_placement(erhe::imgui::Imgui_windows& imgui_windows, erhe::imgui::Imgui_window& new_window)
+{
+    std::string target;
+    const std::vector<erhe::imgui::Imgui_window*>& windows = imgui_windows.get_windows();
+    for (const erhe::imgui::Imgui_window* window : windows) {
+        if ((window == &new_window) || !window->is_window_visible()) {
+            continue;
+        }
+        const Item_tree_window* tree_window = dynamic_cast<const Item_tree_window*>(window);
+        if ((tree_window != nullptr) && tree_window->is_scene_hierarchy()) {
+            target = window->get_title();
+            break;
+        }
+    }
+    new_window.set_initial_placement(target, c_hierarchy_width_ratio, c_hierarchy_height_ratio);
+}
+
 } // namespace editor

src/editor/windows/window_placement.hpp

@@ -21,9 +21,14 @@ namespace editor {
 //    none is currently docked, show it floating, centered, at 33% width x 66%
 //    height of the viewport.
 //
+//  - Hierarchy windows: dock (tab) with an existing scene-hierarchy window. If
+//    none is currently docked, show it floating, centered, at 25% width x 50%
+//    height of the viewport.
+//
 // new_window must already be registered in imgui_windows.
 
 void apply_editor_window_placement    (erhe::imgui::Imgui_windows& imgui_windows, erhe::imgui::Imgui_window& new_window);
 void apply_properties_window_placement(erhe::imgui::Imgui_windows& imgui_windows, erhe::imgui::Imgui_window& new_window);
+void apply_hierarchy_window_placement (erhe::imgui::Imgui_windows& imgui_windows, erhe::imgui::Imgui_window& new_window);
 
 } // namespace editor