wm/win: workaround an xfce bug

xfce will set the WM_TRANSIENT_FOR of some windows to the root window,
which is invalid. Make sure we are robust against this and don't crash.

Signed-off-by: Yuxuan Shui <[email protected]>

src/wm/win.c

@@ -1536,13 +1536,15 @@ win_get_leader_property(struct x_connection *c, struct atom *atoms, xcb_window_t
 	// Read the leader properties
 	if (detect_transient) {
 		leader = wid_get_prop_window(c, wid, atoms->aWM_TRANSIENT_FOR);
+		log_debug("Leader via WM_TRANSIENT_FOR of window %#010x: %#010x", wid, leader);
 	}
 
 	if (detect_client_leader && leader == XCB_NONE) {
 		leader = wid_get_prop_window(c, wid, atoms->aWM_CLIENT_LEADER);
+		log_debug("Leader via WM_CLIENT_LEADER of window %#010x: %#010x", wid, leader);
 	}
 
-	log_trace("window %#010x: leader %#010x", wid, leader);
+	log_debug("window %#010x: leader %#010x", wid, leader);
 	return leader;
 }
 
@@ -1556,6 +1558,12 @@ static struct wm_ref *win_get_leader_raw(session_t *ps, struct win *w, int recur
 		// Leader defaults to client window, or to the window itself if
 		// it doesn't have a client window
 		w->cache_leader = wm_find(ps->wm, w->leader);
+		if (w->cache_leader == wm_root_ref(ps->wm)) {
+			log_warn("Window manager set the leader of window %#010x to "
+			         "root, a broken window manager.",
+			         win_id(w));
+			w->cache_leader = NULL;
+		}
 		if (!w->cache_leader) {
 			w->cache_leader = client_win ?: w->tree_ref;
 		}