Add a theme preview switcher to the 2D viewport

This commit adds a new View submenu that allows switching
between the project theme (default), the editor theme, and
the default theme. The last selected option is stored per
project and is restored when reloading the project.

editor/editor_node.cpp

@@ -482,6 +482,10 @@ void EditorNode::_update_theme(bool p_skip_creation) {
 		}
 	}
 
+	if (CanvasItemEditor::get_singleton()->get_theme_preview() == CanvasItemEditor::THEME_PREVIEW_EDITOR) {
+		update_preview_themes(CanvasItemEditor::THEME_PREVIEW_EDITOR);
+	}
+
 	gui_base->add_theme_style_override("panel", theme->get_stylebox(SNAME("Background"), EditorStringName(EditorStyles)));
 	main_vbox->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT, Control::PRESET_MODE_MINSIZE, theme->get_constant(SNAME("window_border_margin"), EditorStringName(Editor)));
 	main_vbox->add_theme_constant_override("separation", theme->get_constant(SNAME("top_bar_separation"), EditorStringName(Editor)));
@@ -534,6 +538,36 @@ void EditorNode::_update_theme(bool p_skip_creation) {
 	}
 }
 
+void EditorNode::update_preview_themes(int p_mode) {
+	if (!scene_root->is_inside_tree()) {
+		return; // Too early.
+	}
+
+	List<Ref<Theme>> preview_themes;
+
+	switch (p_mode) {
+		case CanvasItemEditor::THEME_PREVIEW_PROJECT:
+			preview_themes.push_back(ThemeDB::get_singleton()->get_project_theme());
+			break;
+
+		case CanvasItemEditor::THEME_PREVIEW_EDITOR:
+			preview_themes.push_back(get_editor_theme());
+			break;
+
+		default:
+			break;
+	}
+
+	preview_themes.push_back(ThemeDB::get_singleton()->get_default_theme());
+
+	ThemeContext *preview_context = ThemeDB::get_singleton()->get_theme_context(scene_root);
+	if (preview_context) {
+		preview_context->set_themes(preview_themes);
+	} else {
+		ThemeDB::get_singleton()->create_theme_context(scene_root, preview_themes);
+	}
+}
+
 void EditorNode::_notification(int p_what) {
 	switch (p_what) {
 		case NOTIFICATION_PROCESS: {
@@ -668,11 +702,9 @@ void EditorNode::_notification(int p_what) {
 
 			_titlebar_resized();
 
-			// Set up a theme context for the 2D preview viewport using the project theme.
-			List<Ref<Theme>> preview_themes;
-			preview_themes.push_back(ThemeDB::get_singleton()->get_project_theme());
-			preview_themes.push_back(ThemeDB::get_singleton()->get_default_theme());
-			ThemeDB::get_singleton()->create_theme_context(scene_root, preview_themes);
+			// Set up a theme context for the 2D preview viewport using the stored preview theme.
+			CanvasItemEditor::ThemePreviewMode theme_preview_mode = (CanvasItemEditor::ThemePreviewMode)(int)EditorSettings::get_singleton()->get_project_metadata("2d_editor", "theme_preview", CanvasItemEditor::THEME_PREVIEW_PROJECT);
+			update_preview_themes(theme_preview_mode);
 
 			/* DO NOT LOAD SCENES HERE, WAIT FOR FILE SCANNING AND REIMPORT TO COMPLETE */
 		} break;

editor/editor_node.h

@@ -851,6 +851,8 @@ public:
 	void stop_child_process(OS::ProcessID p_pid);
 
 	Ref<Theme> get_editor_theme() const { return theme; }
+	void update_preview_themes(int p_mode);
+
 	Ref<Script> get_object_custom_type_base(const Object *p_object) const;
 	StringName get_object_custom_type_name(const Object *p_object) const;
 	Ref<Texture2D> get_object_icon(const Object *p_object, const String &p_fallback = "Object");

editor/plugins/canvas_item_editor_plugin.cpp

@@ -1034,6 +1034,22 @@ void CanvasItemEditor::_on_grid_menu_id_pressed(int p_id) {
 	viewport->queue_redraw();
 }
 
+void CanvasItemEditor::_switch_theme_preview(int p_mode) {
+	view_menu->get_popup()->hide();
+
+	if (theme_preview == p_mode) {
+		return;
+	}
+	theme_preview = (ThemePreviewMode)p_mode;
+	EditorSettings::get_singleton()->set_project_metadata("2d_editor", "theme_preview", theme_preview);
+
+	for (int i = 0; i < THEME_PREVIEW_MAX; i++) {
+		theme_menu->set_item_checked(i, i == theme_preview);
+	}
+
+	EditorNode::get_singleton()->update_preview_themes(theme_preview);
+}
+
 bool CanvasItemEditor::_gui_input_rulers_and_guides(const Ref<InputEvent> &p_event) {
 	EditorUndoRedoManager *undo_redo = EditorUndoRedoManager::get_singleton();
 	Ref<InputEventMouseButton> b = p_event;
@@ -5322,6 +5338,20 @@ CanvasItemEditor::CanvasItemEditor() {
 	p->add_separator();
 	p->add_check_shortcut(ED_SHORTCUT("canvas_item_editor/preview_canvas_scale", TTR("Preview Canvas Scale")), PREVIEW_CANVAS_SCALE);
 
+	theme_menu = memnew(PopupMenu);
+	theme_menu->connect("id_pressed", callable_mp(this, &CanvasItemEditor::_switch_theme_preview));
+	theme_menu->set_name("ThemeMenu");
+	theme_menu->add_radio_check_item(TTR("Project theme"), THEME_PREVIEW_PROJECT);
+	theme_menu->add_radio_check_item(TTR("Editor theme"), THEME_PREVIEW_EDITOR);
+	theme_menu->add_radio_check_item(TTR("Default theme"), THEME_PREVIEW_DEFAULT);
+	p->add_child(theme_menu);
+	p->add_submenu_item(TTR("Preview Theme"), "ThemeMenu");
+
+	theme_preview = (ThemePreviewMode)(int)EditorSettings::get_singleton()->get_project_metadata("2d_editor", "theme_preview", THEME_PREVIEW_PROJECT);
+	for (int i = 0; i < THEME_PREVIEW_MAX; i++) {
+		theme_menu->set_item_checked(i, i == theme_preview);
+	}
+
 	main_menu_hbox->add_child(memnew(VSeparator));
 
 	// Contextual toolbars.

editor/plugins/canvas_item_editor_plugin.h

@@ -325,6 +325,7 @@ private:
 	Button *override_camera_button = nullptr;
 	MenuButton *view_menu = nullptr;
 	PopupMenu *grid_menu = nullptr;
+	PopupMenu *theme_menu = nullptr;
 	HBoxContainer *animation_hb = nullptr;
 	MenuButton *animation_menu = nullptr;
 
@@ -404,6 +405,19 @@ private:
 	void _prepare_grid_menu();
 	void _on_grid_menu_id_pressed(int p_id);
 
+public:
+	enum ThemePreviewMode {
+		THEME_PREVIEW_PROJECT,
+		THEME_PREVIEW_EDITOR,
+		THEME_PREVIEW_DEFAULT,
+
+		THEME_PREVIEW_MAX // The number of options for enumerating.
+	};
+
+private:
+	ThemePreviewMode theme_preview = THEME_PREVIEW_PROJECT;
+	void _switch_theme_preview(int p_mode);
+
 	List<CanvasItem *> _get_edited_canvas_items(bool retrieve_locked = false, bool remove_canvas_item_if_parent_in_selection = true) const;
 	Rect2 _get_encompassing_rect_from_list(List<CanvasItem *> p_list);
 	void _expand_encompassing_rect_using_children(Rect2 &r_rect, const Node *p_node, bool &r_first, const Transform2D &p_parent_xform = Transform2D(), const Transform2D &p_canvas_xform = Transform2D(), bool include_locked_nodes = true);
@@ -558,6 +572,8 @@ public:
 
 	virtual CursorShape get_cursor_shape(const Point2 &p_pos) const override;
 
+	ThemePreviewMode get_theme_preview() const { return theme_preview; }
+
 	EditorSelection *editor_selection = nullptr;
 
 	CanvasItemEditor();