event: tweak ev_reparent_notify

Instead of change window attributes back and forth, calculate the evmask
and set it just once. And also make sure the request is flushed.

(cherry picked from commit baeafb3)
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/event.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: MPL-2.0
 // Copyright (c) 2019, Yuxuan Shui <yshuiv7@gmail.com>
 
+#include <stdint.h>
 #include <stdio.h>
 
 #include <X11/Xlibint.h>
@@ -350,19 +351,14 @@ static inline void ev_reparent_notify(session_t *ps, xcb_reparent_notify_event_t
 		}
 
 		// Reset event mask in case something wrong happens
-		xcb_change_window_attributes(
-		    ps->c.c, ev->window, XCB_CW_EVENT_MASK,
-		    (const uint32_t[]){determine_evmask(ps, ev->window, WIN_EVMODE_UNKNOWN)});
+		uint32_t evmask = determine_evmask(ps, ev->window, WIN_EVMODE_UNKNOWN);
 
 		if (!wid_has_prop(ps, ev->window, ps->atoms->aWM_STATE)) {
 			log_debug("Window %#010x doesn't have WM_STATE property, it is "
 			          "probably not a client window. But we will listen for "
 			          "property change in case it gains one.",
 			          ev->window);
-			xcb_change_window_attributes(
-			    ps->c.c, ev->window, XCB_CW_EVENT_MASK,
-			    (const uint32_t[]){determine_evmask(ps, ev->window, WIN_EVMODE_UNKNOWN) |
-			                       XCB_EVENT_MASK_PROPERTY_CHANGE});
+			evmask |= XCB_EVENT_MASK_PROPERTY_CHANGE;
 		} else {
 			auto w_real_top = find_managed_window_or_parent(ps, ev->parent);
 			if (w_real_top && w_real_top->state != WSTATE_UNMAPPED &&
@@ -386,6 +382,8 @@ static inline void ev_reparent_notify(session_t *ps, xcb_reparent_notify_event_t
 				}
 			}
 		}
+		XCB_AWAIT_VOID(xcb_change_window_attributes, ps->c.c, ev->window,
+		               XCB_CW_EVENT_MASK, (const uint32_t[]){evmask});
 	}
 }