event: fix dumb bug in repair_win

Basically we won't call xcb_damage_subtract if show_all_xerrors is set,
which is very bad.

Fixes that, and also make sure the damage subtract request is flushed in
all branches.

(cherry picked from commit a5826b6)
Fixes: 1307d9e
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/event.c

@@ -585,16 +585,28 @@ static inline void repair_win(session_t *ps, struct managed_win *w) {
 	region_t parts;
 	pixman_region32_init(&parts);
 
+	// If this is the first time this window is damaged, we would redraw the
+	// whole window, so we don't need to fetch the damage region. But we still need
+	// to make sure the X server receives the DamageSubtract request, hence the
+	// `xcb_request_check` here.
+	// Otherwise, we fetch the damage regions. That means we will receive a reply
+	// from the X server, which implies it has received our request.
 	if (!w->ever_damaged) {
-		win_extents(w, &parts);
-		if (!ps->o.show_all_xerrors) {
-			set_ignore_cookie(&ps->c, xcb_damage_subtract(ps->c.c, w->damage,
-			                                              XCB_NONE, XCB_NONE));
+		auto e = xcb_request_check(
+		    ps->c.c, xcb_damage_subtract(ps->c.c, w->damage, XCB_NONE, XCB_NONE));
+		if (e) {
+			if (ps->o.show_all_xerrors) {
+				x_print_error(e->sequence, e->major_code, e->minor_code,
+				              e->error_code);
+			}
+			free(e);
 		}
+		win_extents(w, &parts);
 	} else {
+		auto cookie =
+		    xcb_damage_subtract(ps->c.c, w->damage, XCB_NONE, ps->damaged_region);
 		if (!ps->o.show_all_xerrors) {
-			set_ignore_cookie(&ps->c, xcb_damage_subtract(ps->c.c, w->damage, XCB_NONE,
-			                                              ps->damaged_region));
+			set_ignore_cookie(&ps->c, cookie);
 		}
 		x_fetch_region(&ps->c, ps->damaged_region, &parts);
 		pixman_region32_translate(&parts, w->g.x + w->g.border_width,