XCB: Ignore _NET_WM_PING on the root window

With a _NET_WM_PING message, a window manager can check if an
application still works or if it froze. The WM sends such a message to
the application window and the application sends the message back to the
root window.

Since change Ia04268b0 / commit d8090022f "Move the X11 system tray code
from widgets into XCB-plugin.", Qt selects StructureNotifyMask on the
root window. Due to this, we now also receive replies to _NET_WM_PING
and treat them like a _NET_WM_PING request.

This caused an endless loop as soon as any _NET_WM_PING was seen where
Qt would send a _NET_WM_PING to itself and handle it again and again.

Fix this by ignoring _NET_WM_PING messages that are sent to the root
window. According to EWMH, such messages can only be replies to
_NET_WM_PING and GTK does this, too.

Task-number: QTBUG-32957
Change-Id: I1b0aa682f99b17d633baacc14b18b3adca7a1aba
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/plugins/platforms/xcb/qxcbwindow.cpp b/src/plugins/platforms/xcb/qxcbwindow.cpp
index c95c4d95ba..911db83836 100644
--- a/src/plugins/platforms/xcb/qxcbwindow.cpp
+++ b/src/plugins/platforms/xcb/qxcbwindow.cpp
@@ -1499,6 +1499,9 @@ void QXcbWindow::handleClientMessageEvent(const xcb_client_message_event_t *even
         } else if (event->data.data32[0] == atom(QXcbAtom::WM_TAKE_FOCUS)) {
             connection()->setTime(event->data.data32[1]);
         } else if (event->data.data32[0] == atom(QXcbAtom::_NET_WM_PING)) {
+            if (event->window == m_screen->root())
+                return;
+
             xcb_client_message_event_t reply = *event;
 
             reply.response_type = XCB_CLIENT_MESSAGE;