diff --git a/src/compton.c b/src/compton.c
index 41dab06..f4cc3aa 100644
--- a/src/compton.c
+++ b/src/compton.c
@@ -1272,6 +1272,9 @@ static void handle_new_windows(session_t *ps) {
 			}
 			auto mw = (struct managed_win *)new_w;
 			if (mw->a.map_state == XCB_MAP_STATE_VIEWABLE) {
+				// Have to map immediately instead of queue window update
+				// because we need the window's extent right now.
+				// We can do this because we are in the critical section.
 				map_win_start(ps, mw);
 
 				// This window might be damaged before we called fill_win
diff --git a/src/event.c b/src/event.c
index a92fcf9..873d39d 100644
--- a/src/event.c
+++ b/src/event.c
@@ -285,7 +285,7 @@ static inline void ev_map_notify(session_t *ps, xcb_map_notify_event_t *ev) {
 		return;
 	}
 
-	map_win_start(ps, w);
+	win_queue_update(w, WIN_UPDATE_MAP);
 
 	// FocusIn/Out may be ignored when the window is unmapped, so we must
 	// recheck focus here
diff --git a/src/win.c b/src/win.c
index c83992b..23e6ad6 100644
--- a/src/win.c
+++ b/src/win.c
@@ -1976,6 +1976,7 @@ void win_update_screen(session_t *ps, struct managed_win *w) {
 
 /// Map an already registered window
 void map_win_start(session_t *ps, struct managed_win *w) {
+	assert(ps->server_grabbed);
 	assert(w);
 
 	// Don't care about window mapping if it's an InputOnly window
@@ -2003,6 +2004,7 @@ void map_win_start(session_t *ps, struct managed_win *w) {
 		assert(w);
 	}
 
+	assert(w->state == WSTATE_UNMAPPED);
 	assert((w->flags & WIN_FLAGS_IMAGES_NONE) == WIN_FLAGS_IMAGES_NONE ||
 	       !ps->o.experimental_backends);