QXcbConnection::getTimestamp: do not return stale timestamp

The problem is `PropertyNotify` event generated by
`xcb_delete_property()` at return could be reported as an actual
timestamp at next call due to a missing `state` filter.

Because `PropertyNotify` is generated even if an unchanged property
value is set we can get rid of this delete event easily.

This issue causes observable problems in kwin_x11 (KDE project)
too.

Fixes: QTBUG-56595
Pick-to: 5.15 6.2 6.4 6.5
Change-Id: Ice6cfa934e3769d86e9f7264e369dc5918c8542a
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: JiDe Zhang <zhangjide@uniontech.com>

1 files changed, 2 insertions, 4 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.cpp b/src/plugins/platforms/xcb/qxcbconnection.cpp
index 056075886f..1be95b8414 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.cpp
+++ b/src/plugins/platforms/xcb/qxcbconnection.cpp
@@ -822,8 +822,8 @@ xcb_timestamp_t QXcbConnection::getTimestamp()
 {
     // send a dummy event to myself to get the timestamp from X server.
     xcb_window_t window = rootWindow();
-    xcb_atom_t dummyAtom = atom(QXcbAtom::CLIP_TEMPORARY);
-    xcb_change_property(xcb_connection(), XCB_PROP_MODE_APPEND, window, dummyAtom,
+    xcb_atom_t dummyAtom = atom(QXcbAtom::_QT_GET_TIMESTAMP);
+    xcb_change_property(xcb_connection(), XCB_PROP_MODE_REPLACE, window, dummyAtom,
                         XCB_ATOM_INTEGER, 32, 0, nullptr);
 
     connection()->flush();
@@ -855,8 +855,6 @@ xcb_timestamp_t QXcbConnection::getTimestamp()
     xcb_timestamp_t timestamp = pn->time;
     free(event);
 
-    xcb_delete_property(xcb_connection(), window, dummyAtom);
-
     return timestamp;
 }