xwayland: fix xdg->xwayland drag-and-drop

As struct wlr_drag is destroyed on drop and in the process resets
the focus, a xwayland dnd listener would also reset xwm->drag_focus.

This prevents the xcb replies from being processed and also prevents
the transfer if a compositor would not additionally request new focus
in its wlr_drag destroy handler (which is something usually only done
when in a focus-follows-mouse setting).

This patch creates a new xwm->drop_focus pointer which is a copy of
xwm->drag_focus at drop time. The xcb reply handler and transfer
logic now use the new xwm->drop_focus for their authorization checks.

(cherry picked from commit 546c5d000d)

include/xwayland/xwm.h

@@ -123,6 +123,7 @@ struct wlr_xwm {
 
 	struct wlr_drag *drag;
 	struct wlr_xwayland_surface *drag_focus;
+	struct wlr_xwayland_surface *drop_focus;
 
 	const xcb_query_extension_reply_t *xfixes;
 	const xcb_query_extension_reply_t *xres;
@@ -145,6 +146,7 @@ struct wlr_xwm {
 	struct wl_listener seat_drag_destroy;
 	struct wl_listener seat_drag_source_destroy;
 	struct wl_listener drag_focus_destroy;
+	struct wl_listener drop_focus_destroy;
 };
 
 struct wlr_xwm *xwm_create(struct wlr_xwayland *wlr_xwayland, int wm_fd);

xwayland/selection/dnd.c

@@ -212,10 +212,10 @@ int xwm_handle_selection_client_message(struct wlr_xwm *xwm,
 		bool performed = data->data32[1] & 1;
 		xcb_atom_t action_atom = data->data32[2];
 
-		if (xwm->drag_focus == NULL ||
+		if (xwm->drop_focus == NULL ||
-				target_window != xwm->drag_focus->window_id) {
+				target_window != xwm->drop_focus->window_id) {
 			wlr_log(WLR_DEBUG, "ignoring XdndFinished client message because "
-				"it doesn't match the finished drag focus window ID");
+				"it doesn't match the finished drop focus window ID");
 			return 1;
 		}
 
@@ -241,6 +241,13 @@ static void drag_focus_handle_destroy(struct wl_listener *listener, void *data)
 	xwm_set_drag_focus(xwm, NULL);
 }
 
+static void drop_focus_handle_destroy(struct wl_listener *listener, void *data) {
+	struct wlr_xwm *xwm = wl_container_of(listener, xwm, drop_focus_destroy);
+	wl_list_remove(&xwm->drop_focus_destroy.link);
+	wl_list_init(&xwm->drop_focus_destroy.link);
+	xwm->drop_focus = NULL;
+}
+
 static void xwm_set_drag_focus(struct wlr_xwm *xwm, struct wlr_xwayland_surface *focus) {
 	if (focus == xwm->drag_focus) {
 		return;
@@ -299,6 +306,12 @@ static void seat_handle_drag_drop(struct wl_listener *listener, void *data) {
 	}
 
 	wlr_log(WLR_DEBUG, "Wayland drag dropped over an Xwayland window");
+
+	xwm->drop_focus = xwm->drag_focus;
+	xwm->drop_focus_destroy.notify = drop_focus_handle_destroy;
+	wl_list_remove(&xwm->drop_focus_destroy.link);
+	wl_signal_add(&xwm->drop_focus->events.destroy, &xwm->drop_focus_destroy);
+
 	xwm_dnd_send_drop(xwm, event->time);
 }
 
@@ -329,14 +342,21 @@ static void seat_handle_drag_source_destroy(struct wl_listener *listener,
 	wl_list_remove(&xwm->drag_focus_destroy.link);
 	wl_list_init(&xwm->drag_focus_destroy.link);
 	xwm->drag_focus = NULL;
+
+	wl_list_remove(&xwm->drop_focus_destroy.link);
+	wl_list_init(&xwm->drop_focus_destroy.link);
+	xwm->drop_focus = NULL;
 }
 
 void xwm_seat_handle_start_drag(struct wlr_xwm *xwm, struct wlr_drag *drag) {
 	wl_list_remove(&xwm->drag_focus_destroy.link);
 	wl_list_init(&xwm->drag_focus_destroy.link);
+	wl_list_remove(&xwm->drop_focus_destroy.link);
+	wl_list_init(&xwm->drop_focus_destroy.link);
 
 	xwm->drag = drag;
 	xwm->drag_focus = NULL;
+	xwm->drop_focus = NULL;
 
 	if (drag != NULL) {
 		wl_signal_add(&drag->events.focus, &xwm->seat_drag_focus);
@@ -357,6 +377,7 @@ void xwm_seat_handle_start_drag(struct wlr_xwm *xwm, struct wlr_drag *drag) {
 void xwm_seat_unlink_drag_handlers(struct wlr_xwm *xwm) {
 	wl_list_remove(&xwm->seat_drag_source_destroy.link);
 	wl_list_remove(&xwm->drag_focus_destroy.link);
+	wl_list_remove(&xwm->drop_focus_destroy.link);
 
 	if (!xwm->drag) {
 		return;

xwayland/selection/outgoing.c

@@ -410,8 +410,11 @@ void xwm_handle_selection_request(struct wlr_xwm *xwm,
 		return;
 	}
 
+	bool dnd_allowed = selection == &xwm->dnd_selection
+		&& (xwm->drag_focus != NULL || xwm->drop_focus != NULL);
+
 	// No xwayland surface focused, deny access to clipboard
-	if (xwm->focus_surface == NULL && xwm->drag_focus == NULL) {
+	if (xwm->focus_surface == NULL && !dnd_allowed) {
 		if (wlr_log_get_verbosity() >= WLR_DEBUG) {
 			char *selection_name = xwm_get_atom_name(xwm, selection->atom);
 			wlr_log(WLR_DEBUG, "denying read access to selection %u (%s): "

xwayland/xwm.c

@@ -2177,6 +2177,7 @@ struct wlr_xwm *xwm_create(struct wlr_xwayland *xwayland, int wm_fd) {
 	wl_list_init(&xwm->pending_startup_ids);
 	wl_list_init(&xwm->seat_drag_source_destroy.link);
 	wl_list_init(&xwm->drag_focus_destroy.link);
+	wl_list_init(&xwm->drop_focus_destroy.link);
 
 	xwm->ping_timeout = 10000;