xcb: Flush Display when processing xcb events

Only calling xcb_flush() is not enough when users use xlib calls
to paint with the Display, for example in QWidget::paintEvent(),
and not call XFlush().

This fixes a regression since Qt 4.x.

In Qt 4, we use XNextEvent() in x11 event dispatcher which
implicitly called XFlush(). In Qt 5 and 6, we use xcb calls.

See also https://www.x.org/releases/X11R7.5/doc/man/man3/XFlush.3.html

Most client applications need not use this function because the
output buffer is automatically flushed as needed by calls to
XPending, XNextEvent, and XWindowEvent.

Pick-to: 6.4 6.2 5.15
Fixes: QTBUG-46681
Fixes: QTBUG-100085
Change-Id: I08dc0800ad0e23f0c2293d0d4e4bd29d92155752
Reviewed-by: JiDe Zhang <zhangjide@uniontech.com>
Reviewed-by: Liang Qi <liang.qi@qt.io>

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -35,6 +35,10 @@
 #undef explicit
 #include <xcb/xinput.h>
 
+#if QT_CONFIG(xcb_xlib)
+#include "qt_xlib_wrapper.h"
+#endif
+
 QT_BEGIN_NAMESPACE
 
 using namespace Qt::StringLiterals;
@@ -1090,6 +1094,10 @@ void QXcbConnection::processXcbEvents(QEventLoop::ProcessEventsFlags flags)
         m_eventQueue->flushBufferedEvents();
     }
 
+#if QT_CONFIG(xcb_xlib)
+    qt_XFlush(static_cast<Display *>(xlib_display()));
+#endif
+
     xcb_flush(xcb_connection());
 }