core: fix debug window check in paint_preprocess

Fixes #704

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/event.c

@@ -270,6 +270,11 @@ static inline void ev_destroy_notify(session_t *ps, xcb_destroy_notify_event_t *
 	if (w != NULL) {
 		auto _ attr_unused = destroy_win_start(ps, w);
 	} else if (mw != NULL) {
+		// XXX the hope here is the WM will destroy the frame window
+		// quickly after the client window is destroyed. Otherwise a
+		// frame window without a client window would linger around. Who
+		// knows what kind of bugs it could cause. This has caused at
+		// least one: #704
 		win_unmark_client(ps, mw);
 		win_set_flags(mw, WIN_FLAGS_CLIENT_STALE);
 		ps->pending_updates = true;

src/picom.c

@@ -980,8 +980,9 @@ static bool paint_preprocess(session_t *ps, bool *fade_running, bool *animation,
 		if (w->state == WSTATE_UNMAPPED) {
 			log_trace("|- is unmapped");
 			to_paint = false;
-		} else if (unlikely(w->base.id == ps->debug_window ||
-		                    w->client_win == ps->debug_window)) {
+		} else if (unlikely(ps->debug_window != XCB_NONE) &&
+		           (w->base.id == ps->debug_window ||
+		            w->client_win == ps->debug_window)) {
 			log_trace("|- is the debug window");
 			to_paint = false;
 		} else if (!w->ever_damaged && w->state != WSTATE_UNMAPPING &&