XCB: Force XSync after creating Window

Make sure that dpy->request == dpy->last_request_read after setting
up a new window. If we don't do this, last_request_read might never
be updated until the difference hits a limit that can lead to hangs
in the application (see e.g. QTCREATORBUG-8373).

Task-number: QTBUG-29106
Change-Id: I390493ca6f966dc105d3ea3a2c48abec01177bc2
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -382,6 +382,11 @@ void QXcbWindow::create()
     if (window()->flags() & Qt::WindowTransparentForInput)
         setTransparentForMouseEvents(true);
 
+#ifdef XCB_USE_XLIB
+    // force sync to read outstanding requests - see QTBUG-29106
+    XSync(DISPLAY_FROM_XCB(m_screen), false);
+#endif
+
 #ifndef QT_NO_DRAGANDDROP
     connection()->drag()->dndEnable(this, true);
 #endif