event: add damage for reparented windows

See code comments for details.

Fixes #1081

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

src/event.c

@@ -353,6 +353,16 @@ static inline void ev_reparent_notify(session_t *ps, xcb_reparent_notify_event_t
 				if (!ret && w->managed) {
 					auto mw = (struct managed_win *)w;
 					CHECK(win_skip_fading(ps, mw));
+					// Usually, damage for unmapped windows
+					// are added in `paint_preprocess`, when
+					// a window was painted before and isn't
+					// anymore. But since we are reparenting
+					// the window here, we would lose track
+					// of the `to_paint` information. So we
+					// just add damage here.
+					if (mw->to_paint) {
+						add_damage_from_win(ps, mw);
+					}
 				}
 			}
 		}