[Window] Expose `start_drag` and `start_resize` methods (for both native and embedded windows).

doc/classes/Window.xml

@@ -557,6 +557,19 @@
 				Makes the [Window] appear. This enables interactions with the [Window] and doesn't change any of its property other than visibility (unlike e.g. [method popup]).
 			</description>
 		</method>
+		<method name="start_drag">
+			<return type="void" />
+			<description>
+				Starts an interactive drag operation on the window, using the current mouse position. Call this method when handling a mouse button being pressed to simulate a pressed event on the window's title bar. Using this method allows the window to participate in space switching, tiling, and other system features.
+			</description>
+		</method>
+		<method name="start_resize">
+			<return type="void" />
+			<param index="0" name="edge" type="int" enum="DisplayServer.WindowResizeEdge" />
+			<description>
+				Starts an interactive resize operation on the window, using the current mouse position. Call this method when handling a mouse button being pressed to simulate a pressed event on the window's edge.
+			</description>
+		</method>
 	</methods>
 	<members>
 		<member name="always_on_top" type="bool" setter="set_flag" getter="get_flag" default="false">

platform/linuxbsd/x11/display_server_x11.cpp

@@ -5496,6 +5496,10 @@ void DisplayServerX11::window_start_drag(WindowID p_window) {
 	ERR_FAIL_COND(!windows.has(p_window));
 	WindowData &wd = windows[p_window];
 
+	if (wd.embed_parent) {
+		return; // Embedded window.
+	}
+
 	XClientMessageEvent m;
 	memset(&m, 0, sizeof(m));
 
@@ -5532,6 +5536,10 @@ void DisplayServerX11::window_start_resize(WindowResizeEdge p_edge, WindowID p_w
 	ERR_FAIL_COND(!windows.has(p_window));
 	WindowData &wd = windows[p_window];
 
+	if (wd.embed_parent) {
+		return; // Embedded window.
+	}
+
 	XClientMessageEvent m;
 	memset(&m, 0, sizeof(m));
 

platform/windows/display_server_windows.cpp

@@ -3990,6 +3990,10 @@ void DisplayServerWindows::window_start_drag(WindowID p_window) {
 	ERR_FAIL_COND(!windows.has(p_window));
 	WindowData &wd = windows[p_window];
 
+	if (wd.parent_hwnd) {
+		return; // Embedded window.
+	}
+
 	ReleaseCapture();
 
 	POINT coords;
@@ -4006,6 +4010,10 @@ void DisplayServerWindows::window_start_resize(WindowResizeEdge p_edge, WindowID
 	ERR_FAIL_COND(!windows.has(p_window));
 	WindowData &wd = windows[p_window];
 
+	if (wd.parent_hwnd) {
+		return; // Embedded window.
+	}
+
 	ReleaseCapture();
 
 	POINT coords;

scene/main/viewport.cpp

@@ -2956,6 +2956,47 @@ bool Viewport::_sub_windows_forward_input(const Ref<InputEvent> &p_event) {
 	return true;
 }
 
+void Viewport::_window_start_drag(Window *p_window) {
+	int index = _sub_window_find(p_window);
+	ERR_FAIL_COND(index == -1);
+
+	SubWindow sw = gui.sub_windows.write[index];
+
+	if (gui.subwindow_focused != sw.window) {
+		// Refocus.
+		_sub_window_grab_focus(sw.window);
+	}
+
+	gui.subwindow_drag = SUB_WINDOW_DRAG_MOVE;
+	gui.subwindow_drag_from = get_mouse_position();
+	gui.subwindow_drag_pos = sw.window->get_position();
+	gui.currently_dragged_subwindow = sw.window;
+
+	_sub_window_update(sw.window);
+}
+
+void Viewport::_window_start_resize(SubWindowResize p_edge, Window *p_window) {
+	int index = _sub_window_find(p_window);
+	ERR_FAIL_COND(index == -1);
+
+	SubWindow sw = gui.sub_windows.write[index];
+	Rect2i r = Rect2i(sw.window->get_position(), sw.window->get_size());
+
+	if (gui.subwindow_focused != sw.window) {
+		// Refocus.
+		_sub_window_grab_focus(sw.window);
+	}
+
+	gui.subwindow_drag = SUB_WINDOW_DRAG_RESIZE;
+	gui.subwindow_resize_mode = p_edge;
+	gui.subwindow_resize_from_rect = r;
+	gui.subwindow_drag_from = get_mouse_position();
+	gui.subwindow_drag_pos = sw.window->get_position();
+	gui.currently_dragged_subwindow = sw.window;
+
+	_sub_window_update(sw.window);
+}
+
 void Viewport::_update_mouse_over() {
 	// Update gui.mouse_over and gui.subwindow_over in all Viewports.
 	// Send necessary mouse_enter/mouse_exit signals and the MOUSE_ENTER/MOUSE_EXIT notifications for every Viewport in the SceneTree.

scene/main/viewport.h

@@ -488,6 +488,9 @@ private:
 	void _process_dirty_canvas_parent_orders();
 	void _propagate_world_2d_changed(Node *p_node);
 
+	void _window_start_drag(Window *p_window);
+	void _window_start_resize(SubWindowResize p_edge, Window *p_window);
+
 protected:
 	bool _set_size(const Size2i &p_size, const Size2i &p_size_2d_override, bool p_allocated);
 

scene/main/window.cpp

@@ -1998,6 +1998,54 @@ bool Window::has_focus() const {
 	return focused;
 }
 
+void Window::start_drag() {
+	ERR_MAIN_THREAD_GUARD;
+	if (window_id != DisplayServer::INVALID_WINDOW_ID) {
+		DisplayServer::get_singleton()->window_start_drag(window_id);
+	} else if (embedder) {
+		embedder->_window_start_drag(this);
+	}
+}
+
+void Window::start_resize(DisplayServer::WindowResizeEdge p_edge) {
+	ERR_MAIN_THREAD_GUARD;
+	if (get_flag(FLAG_RESIZE_DISABLED)) {
+		return;
+	}
+	if (window_id != DisplayServer::INVALID_WINDOW_ID) {
+		DisplayServer::get_singleton()->window_start_resize(p_edge, window_id);
+	} else if (embedder) {
+		switch (p_edge) {
+			case DisplayServer::WINDOW_EDGE_TOP_LEFT: {
+				embedder->_window_start_resize(Viewport::SUB_WINDOW_RESIZE_TOP_LEFT, this);
+			} break;
+			case DisplayServer::WINDOW_EDGE_TOP: {
+				embedder->_window_start_resize(Viewport::SUB_WINDOW_RESIZE_TOP, this);
+			} break;
+			case DisplayServer::WINDOW_EDGE_TOP_RIGHT: {
+				embedder->_window_start_resize(Viewport::SUB_WINDOW_RESIZE_TOP_RIGHT, this);
+			} break;
+			case DisplayServer::WINDOW_EDGE_LEFT: {
+				embedder->_window_start_resize(Viewport::SUB_WINDOW_RESIZE_LEFT, this);
+			} break;
+			case DisplayServer::WINDOW_EDGE_RIGHT: {
+				embedder->_window_start_resize(Viewport::SUB_WINDOW_RESIZE_RIGHT, this);
+			} break;
+			case DisplayServer::WINDOW_EDGE_BOTTOM_LEFT: {
+				embedder->_window_start_resize(Viewport::SUB_WINDOW_RESIZE_BOTTOM_LEFT, this);
+			} break;
+			case DisplayServer::WINDOW_EDGE_BOTTOM: {
+				embedder->_window_start_resize(Viewport::SUB_WINDOW_RESIZE_BOTTOM, this);
+			} break;
+			case DisplayServer::WINDOW_EDGE_BOTTOM_RIGHT: {
+				embedder->_window_start_resize(Viewport::SUB_WINDOW_RESIZE_BOTTOM_RIGHT, this);
+			} break;
+			default:
+				break;
+		}
+	}
+}
+
 Rect2i Window::get_usable_parent_rect() const {
 	ERR_READ_THREAD_GUARD_V(Rect2i());
 	ERR_FAIL_COND_V(!is_inside_tree(), Rect2());
@@ -2888,6 +2936,9 @@ void Window::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("has_focus"), &Window::has_focus);
 	ClassDB::bind_method(D_METHOD("grab_focus"), &Window::grab_focus);
 
+	ClassDB::bind_method(D_METHOD("start_drag"), &Window::start_drag);
+	ClassDB::bind_method(D_METHOD("start_resize", "edge"), &Window::start_resize);
+
 	ClassDB::bind_method(D_METHOD("set_ime_active", "active"), &Window::set_ime_active);
 	ClassDB::bind_method(D_METHOD("set_ime_position", "position"), &Window::set_ime_position);
 

scene/main/window.h

@@ -401,6 +401,9 @@ public:
 	void grab_focus();
 	bool has_focus() const;
 
+	void start_drag();
+	void start_resize(DisplayServer::WindowResizeEdge p_edge);
+
 	Rect2i get_usable_parent_rect() const;
 
 	// Internationalization.