Implement window_start_drag on Windows and Linux.

doc/classes/DisplayServer.xml

@@ -1836,7 +1836,7 @@
 			<param index="0" name="window_id" type="int" default="0" />
 			<description>
 				Starts a drag operation on the window with the given [param window_id], 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.
-				[b]Note:[/b] This method is implemented only on macOS.
+				[b]Note:[/b] This method is implemented on Linux(X11/Wayland), macOS, and Windows.
 			</description>
 		</method>
 	</methods>

platform/linuxbsd/wayland/display_server_wayland.cpp

@@ -986,6 +986,12 @@ DisplayServer::VSyncMode DisplayServerWayland::window_get_vsync_mode(DisplayServ
 	return DisplayServer::VSYNC_ENABLED;
 }
 
+void DisplayServerWayland::window_start_drag(WindowID p_window) {
+	MutexLock mutex_lock(wayland_thread.mutex);
+
+	wayland_thread.window_start_drag(p_window);
+}
+
 void DisplayServerWayland::cursor_set_shape(CursorShape p_shape) {
 	ERR_FAIL_INDEX(p_shape, CURSOR_MAX);
 

platform/linuxbsd/wayland/display_server_wayland.h

@@ -272,6 +272,8 @@ public:
 	virtual void window_set_vsync_mode(DisplayServer::VSyncMode p_vsync_mode, WindowID p_window_id = MAIN_WINDOW_ID) override;
 	virtual DisplayServer::VSyncMode window_get_vsync_mode(WindowID p_window_id) const override;
 
+	virtual void window_start_drag(WindowID p_window = MAIN_WINDOW_ID) override;
+
 	virtual void cursor_set_shape(CursorShape p_shape) override;
 	virtual CursorShape cursor_get_shape() const override;
 	virtual void cursor_set_custom_image(const Ref<Resource> &p_cursor, CursorShape p_shape, const Vector2 &p_hotspot) override;

platform/linuxbsd/wayland/wayland_thread.cpp

@@ -3338,6 +3338,22 @@ void WaylandThread::beep() const {
 	}
 }
 
+void WaylandThread::window_start_drag(DisplayServer::WindowID p_window_id) {
+	// TODO: Use window IDs for multiwindow support.
+	WindowState &ws = main_window;
+	SeatState *ss = wl_seat_get_seat_state(wl_seat_current);
+
+	if (ss && ws.xdg_toplevel) {
+		xdg_toplevel_move(ws.xdg_toplevel, ss->wl_seat, ss->pointer_data.button_serial);
+	}
+
+#ifdef LIBDECOR_ENABLED
+	if (ws.libdecor_frame) {
+		libdecor_frame_move(ws.libdecor_frame, ss->wl_seat, ss->pointer_data.button_serial);
+	}
+#endif
+}
+
 void WaylandThread::window_set_max_size(DisplayServer::WindowID p_window_id, const Size2i &p_size) {
 	// TODO: Use window IDs for multiwindow support.
 	WindowState &ws = main_window;

platform/linuxbsd/wayland/wayland_thread.h

@@ -971,6 +971,8 @@ public:
 	// Optional - requires xdg_activation_v1
 	void window_request_attention(DisplayServer::WindowID p_window_id);
 
+	void window_start_drag(DisplayServer::WindowID p_window_id);
+
 	// Optional - require idle_inhibit_unstable_v1
 	void window_set_idle_inhibition(DisplayServer::WindowID p_window_id, bool p_enable);
 	bool window_get_idle_inhibition(DisplayServer::WindowID p_window_id) const;

platform/linuxbsd/x11/display_server_x11.cpp

@@ -69,6 +69,8 @@
 #define _NET_WM_STATE_REMOVE 0L // remove/unset property
 #define _NET_WM_STATE_ADD 1L // add/set property
 
+#define _NET_WM_MOVERESIZE_MOVE 8L
+
 // 2.2 is the first release with multitouch
 #define XINPUT_CLIENT_VERSION_MAJOR 2
 #define XINPUT_CLIENT_VERSION_MINOR 2
@@ -5422,6 +5424,40 @@ DisplayServer::VSyncMode DisplayServerX11::window_get_vsync_mode(WindowID p_wind
 	return DisplayServer::VSYNC_ENABLED;
 }
 
+void DisplayServerX11::window_start_drag(WindowID p_window) {
+	_THREAD_SAFE_METHOD_
+
+	ERR_FAIL_COND(!windows.has(p_window));
+	WindowData &wd = windows[p_window];
+
+	XClientMessageEvent m;
+	memset(&m, 0, sizeof(m));
+
+	XUngrabPointer(x11_display, CurrentTime);
+
+	Window root_return, child_return;
+	int root_x, root_y, win_x, win_y;
+	unsigned int mask_return;
+
+	Bool xquerypointer_result = XQueryPointer(x11_display, wd.x11_window, &root_return, &child_return, &root_x, &root_y, &win_x, &win_y, &mask_return);
+
+	m.type = ClientMessage;
+	m.window = wd.x11_window;
+	m.message_type = XInternAtom(x11_display, "_NET_WM_MOVERESIZE", True);
+	m.format = 32;
+	if (xquerypointer_result) {
+		m.data.l[0] = root_x;
+		m.data.l[1] = root_y;
+		m.data.l[3] = Button1;
+	}
+	m.data.l[2] = _NET_WM_MOVERESIZE_MOVE;
+	m.data.l[4] = 1; // Source - normal application.
+
+	XSendEvent(x11_display, DefaultRootWindow(x11_display), False, SubstructureRedirectMask | SubstructureNotifyMask, (XEvent *)&m);
+
+	XSync(x11_display, 0);
+}
+
 Vector<String> DisplayServerX11::get_rendering_drivers_func() {
 	Vector<String> drivers;
 

platform/linuxbsd/x11/display_server_x11.h

@@ -512,6 +512,8 @@ public:
 	virtual void window_set_vsync_mode(DisplayServer::VSyncMode p_vsync_mode, WindowID p_window = MAIN_WINDOW_ID) override;
 	virtual DisplayServer::VSyncMode window_get_vsync_mode(WindowID p_vsync_mode) const override;
 
+	virtual void window_start_drag(WindowID p_window = MAIN_WINDOW_ID) override;
+
 	virtual void cursor_set_shape(CursorShape p_shape) override;
 	virtual CursorShape cursor_get_shape() const override;
 	virtual void cursor_set_custom_image(const Ref<Resource> &p_cursor, CursorShape p_shape, const Vector2 &p_hotspot) override;

platform/windows/display_server_windows.cpp

@@ -3803,6 +3803,21 @@ DisplayServer::VSyncMode DisplayServerWindows::window_get_vsync_mode(WindowID p_
 	return DisplayServer::VSYNC_ENABLED;
 }
 
+void DisplayServerWindows::window_start_drag(WindowID p_window) {
+	_THREAD_SAFE_METHOD_
+
+	ERR_FAIL_COND(!windows.has(p_window));
+	WindowData &wd = windows[p_window];
+
+	ReleaseCapture();
+
+	POINT coords;
+	GetCursorPos(&coords);
+	ScreenToClient(wd.hWnd, &coords);
+
+	SendMessage(wd.hWnd, WM_SYSCOMMAND, SC_MOVE | HTCAPTION, MAKELPARAM(coords.x, coords.y));
+}
+
 void DisplayServerWindows::set_context(Context p_context) {
 }
 

platform/windows/display_server_windows.h

@@ -805,6 +805,8 @@ public:
 	virtual void window_set_vsync_mode(DisplayServer::VSyncMode p_vsync_mode, WindowID p_window = MAIN_WINDOW_ID) override;
 	virtual DisplayServer::VSyncMode window_get_vsync_mode(WindowID p_vsync_mode) const override;
 
+	virtual void window_start_drag(WindowID p_window = MAIN_WINDOW_ID) override;
+
 	virtual void cursor_set_shape(CursorShape p_shape) override;
 	virtual CursorShape cursor_get_shape() const override;
 	virtual void cursor_set_custom_image(const Ref<Resource> &p_cursor, CursorShape p_shape = CURSOR_ARROW, const Vector2 &p_hotspot = Vector2()) override;