diff --git a/i3bar/include/xcb_atoms.def b/i3bar/include/xcb_atoms.def
index 65a147c47..453dd0ceb 100644
--- a/i3bar/include/xcb_atoms.def
+++ b/i3bar/include/xcb_atoms.def
@@ -9,4 +9,5 @@ ATOM_DO(_NET_SYSTEM_TRAY_OPCODE)
 ATOM_DO(_NET_SYSTEM_TRAY_COLORS)
 ATOM_DO(_XEMBED_INFO)
 ATOM_DO(_XEMBED)
+ATOM_DO(I3_SYNC)
 #undef ATOM_DO
diff --git a/i3bar/src/xcb.c b/i3bar/src/xcb.c
index fed969df7..dfa66ee2d 100644
--- a/i3bar/src/xcb.c
+++ b/i3bar/src/xcb.c
@@ -678,8 +678,26 @@ static void configure_trayclients(void) {
  *
  */
 static void handle_client_message(xcb_client_message_event_t *event) {
-    if (event->type == atoms[_NET_SYSTEM_TRAY_OPCODE] &&
-        event->format == 32) {
+    if (event->type == atoms[I3_SYNC]) {
+        xcb_window_t window = event->data.data32[0];
+        uint32_t rnd = event->data.data32[1];
+        DLOG("[i3 sync protocol] Forwarding random value %d, X11 window 0x%08x to i3\n", rnd, window);
+
+        void *reply = scalloc(32, 1);
+        xcb_client_message_event_t *ev = reply;
+
+        ev->response_type = XCB_CLIENT_MESSAGE;
+        ev->window = window;
+        ev->type = atoms[I3_SYNC];
+        ev->format = 32;
+        ev->data.data32[0] = window;
+        ev->data.data32[1] = rnd;
+
+        xcb_send_event(conn, false, xcb_root, XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT, (char *)ev);
+        xcb_flush(conn);
+        free(reply);
+    } else if (event->type == atoms[_NET_SYSTEM_TRAY_OPCODE] &&
+               event->format == 32) {
         DLOG("_NET_SYSTEM_TRAY_OPCODE received\n");
         /* event->data.data32[0] is the timestamp */
         uint32_t op = event->data.data32[1];
diff --git a/testcases/lib/i3test/XTEST.pm b/testcases/lib/i3test/XTEST.pm
index 5c1dfe13a..4c464c5e3 100644
--- a/testcases/lib/i3test/XTEST.pm
+++ b/testcases/lib/i3test/XTEST.pm
@@ -14,6 +14,7 @@ use ExtUtils::PkgConfig;
 use Exporter ();
 our @EXPORT = qw(
     inlinec_connect
+    xtest_sync_with
     xtest_sync_with_i3
     set_xkb_group
     xtest_key_press
@@ -118,7 +119,7 @@ bool inlinec_connect() {
     return true;
 }
 
-void xtest_sync_with_i3() {
+void xtest_sync_with(int window) {
     xcb_client_message_event_t ev;
     memset(&ev, '\0', sizeof(xcb_client_message_event_t));
 
@@ -131,7 +132,7 @@ void xtest_sync_with_i3() {
     ev.data.data32[0] = sync_window;
     ev.data.data32[1] = nonce;
 
-    xcb_send_event(conn, false, root_window, XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT, (char *)&ev);
+    xcb_send_event(conn, false, (xcb_window_t)window, XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT, (char *)&ev);
     xcb_flush(conn);
 
     xcb_generic_event_t *event = NULL;
@@ -176,6 +177,10 @@ void xtest_sync_with_i3() {
     free(event);
 }
 
+void xtest_sync_with_i3() {
+    xtest_sync_with((int)root_window);
+}
+
 // NOTE: while |group| should be a uint8_t, Inline::C will not define the
 // function unless we use an int.
 bool set_xkb_group(int group) {
@@ -287,6 +292,11 @@ Sends a ButtonRelease event via XTEST, with the specified C<$button>.
 
 Returns false when there was an X11 error, true otherwise.
 
+=head2 xtest_sync_with($window)
+
+Ensures the specified window has processed all X11 events which were triggered
+by this module, provided the window response to the i3 sync protocol.
+
 =head2 xtest_sync_with_i3()
 
 Ensures i3 has processed all X11 events which were triggered by this module.
diff --git a/testcases/t/525-i3bar-mouse-bindings.t b/testcases/t/525-i3bar-mouse-bindings.t
index 1dfa6aa71..d3216d974 100644
--- a/testcases/t/525-i3bar-mouse-bindings.t
+++ b/testcases/t/525-i3bar-mouse-bindings.t
@@ -61,7 +61,7 @@ sub i3bar_present {
     for my $node (@{$nodes}) {
 	my $props = $node->{window_properties};
 	if (defined($props) && $props->{class} eq 'i3bar') {
-	    return 1;
+	    return $node->{window};
 	}
     }
 
@@ -73,13 +73,17 @@ sub i3bar_present {
     return i3bar_present(\@children);
 }
 
-if (i3bar_present($i3->get_tree->recv->{nodes})) {
+my $i3bar_window = i3bar_present($i3->get_tree->recv->{nodes});
+if ($i3bar_window) {
     ok(1, 'i3bar present');
 } else {
     my $con = $cv->recv;
     ok($con, 'i3bar appeared');
+    $i3bar_window = $con->{window};
 }
 
+diag('i3bar window = ' . $i3bar_window);
+
 my $left = open_window;
 my $right = open_window;
 sync_with_i3;
@@ -99,7 +103,7 @@ subtest 'button 1 moves focus left', \&focus_subtest,
     sub {
 	xtest_button_press(1, 3, 3);
 	xtest_button_release(1, 3, 3);
-	xtest_sync_with_i3;
+	xtest_sync_with($i3bar_window);
     },
     [ $left->{id} ],
     'button 1 moves focus left';
@@ -108,7 +112,7 @@ subtest 'button 2 moves focus right', \&focus_subtest,
     sub {
 	xtest_button_press(2, 3, 3);
 	xtest_button_release(2, 3, 3);
-	xtest_sync_with_i3;
+	xtest_sync_with($i3bar_window);
     },
     [ $right->{id} ],
     'button 2 moves focus right';
@@ -117,7 +121,7 @@ subtest 'button 3 moves focus left', \&focus_subtest,
     sub {
 	xtest_button_press(3, 3, 3);
 	xtest_button_release(3, 3, 3);
-	xtest_sync_with_i3;
+	xtest_sync_with($i3bar_window);
     },
     [ $left->{id} ],
     'button 3 moves focus left';
@@ -126,7 +130,7 @@ subtest 'button 4 moves focus right', \&focus_subtest,
     sub {
 	xtest_button_press(4, 3, 3);
 	xtest_button_release(4, 3, 3);
-	xtest_sync_with_i3;
+	xtest_sync_with($i3bar_window);
     },
     [ $right->{id} ],
     'button 4 moves focus right';
@@ -135,7 +139,7 @@ subtest 'button 5 moves focus left', \&focus_subtest,
     sub {
 	xtest_button_press(5, 3, 3);
 	xtest_button_release(5, 3, 3);
-	xtest_sync_with_i3;
+	xtest_sync_with($i3bar_window);
     },
     [ $left->{id} ],
     'button 5 moves focus left';