[REG] QXcbConnection fails due to "Unsupported extension used" when querying XInputExtension

$ Xvfb :12 &
$ DISPLAY=:12 gdb ./test_Html_formatting
...
(gdb) catch syscall write
...
[after a couple of iterations)
...
The X11 connection broke: Unsupported extension used (code 2)
XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":12"
Catchpoint 1 (returned from syscall write), 0x00007ffff36bb97d in write () at ../sysdeps/unix/syscall-template.S:81
81      in ../sysdeps/unix/syscall-template.S
(gdb) bt
#0  0x00007ffff36bb97d in write () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007ffff3656b13 in _IO_new_file_write (f=0x7ffff397d000 <_IO_2_1_stderr_>, data=<optimized out>, n=78) at fileops.c:1253
#2  0x00007ffff36569f2 in new_do_write (fp=fp@entry=0x7ffff397d000 <_IO_2_1_stderr_>, 
    data=data@entry=0x7fffffffa930 "XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server \":12\"\r\n", to_do=to_do@entry=78) at fileops.c:530
#3  0x00007ffff365747e in _IO_new_file_xsputn (n=78, data=<optimized out>, f=0x7ffff397d000 <_IO_2_1_stderr_>) at fileops.c:1335
#4  _IO_new_file_xsputn (f=0x7ffff397d000 <_IO_2_1_stderr_>, data=<optimized out>, n=78) at fileops.c:1269
#5  0x00007ffff362bf6b in buffered_vfprintf (s=s@entry=0x7ffff397d000 <_IO_2_1_stderr_>, format=format@entry=0x7ffff1fa4c88 "XIO:  fatal IO error %d (%s) on X server \"%s\"\r\n", 
    args=args@entry=0x7fffffffcf38) at vfprintf.c:2332
#6  0x00007ffff3626cfe in _IO_vfprintf_internal (s=s@entry=0x7ffff397d000 <_IO_2_1_stderr_>, format=format@entry=0x7ffff1fa4c88 "XIO:  fatal IO error %d (%s) on X server \"%s\"\r\n", 
    ap=ap@entry=0x7fffffffcf38) at vfprintf.c:1289
#7  0x00007ffff36d827d in ___fprintf_chk (fp=0x7ffff397d000 <_IO_2_1_stderr_>, flag=flag@entry=1, format=format@entry=0x7ffff1fa4c88 "XIO:  fatal IO error %d (%s) on X server \"%s\"\r\n")
    at fprintf_chk.c:35
#8  0x00007ffff1f2919d in fprintf (__fmt=0x7ffff1fa4c88 "XIO:  fatal IO error %d (%s) on X server \"%s\"\r\n", __stream=<optimized out>) at /usr/include/bits/stdio2.h:97
#9  _XDefaultIOError (dpy=0x555555782b40) at /var/tmp/portage/x11-libs/libX11-1.6.2/work/libX11-1.6.2/src/XlibInt.c:1278
#10 0x00007fffeacb1a9d in ioErrorHandler (dpy=0x555555782b40) at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/plugins/platforms/xcb/qxcbconnection.cpp:116
#11 0x00007ffff1f29462 in _XIOError (dpy=dpy@entry=0x555555782b40) at /var/tmp/portage/x11-libs/libX11-1.6.2/work/libX11-1.6.2/src/XlibInt.c:1498
#12 0x00007ffff1f260ef in require_socket (dpy=0x555555782b40) at /var/tmp/portage/x11-libs/libX11-1.6.2/work/libX11-1.6.2/src/xcb_io.c:70
#13 0x00007ffff1f269ad in _XFlush (dpy=0x555555782b40) at /var/tmp/portage/x11-libs/libX11-1.6.2/work/libX11-1.6.2/src/xcb_io.c:511
#14 0x00007ffff1f29cc5 in _XGetRequest (dpy=dpy@entry=0x555555782b40, type=type@entry=98 'b', len=len@entry=8)
    at /var/tmp/portage/x11-libs/libX11-1.6.2/work/libX11-1.6.2/src/XlibInt.c:1735
#15 0x00007ffff1f1ae61 in XQueryExtension (dpy=0x555555782b40, name=0x7fffead03437 "XInputExtension", major_opcode=0x555555782768, first_event=0x55555578276c, first_error=0x555555782770)
    at /var/tmp/portage/x11-libs/libX11-1.6.2/work/libX11-1.6.2/src/QuExt.c:44
#16 0x00007fffeace0d7a in QXcbConnection::initializeXInput2 (this=this@entry=0x555555782750)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/plugins/platforms/xcb/qxcbconnection_xi2.cpp:72
#17 0x00007fffeacb7bad in QXcbConnection::QXcbConnection (this=0x555555782750, nativeInterface=0x5555557826a0, canGrabServer=<optimized out>, displayName=0x0)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/plugins/platforms/xcb/qxcbconnection.cpp:510
#18 0x00007fffeacbc23f in QXcbIntegration::QXcbIntegration (this=0x555555778fa0, parameters=..., argc=<optimized out>, argv=0x7fffffffd978)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/plugins/platforms/xcb/qxcbintegration.cpp:170
#19 0x00007ffff7ff540e in QXcbIntegrationPlugin::create (this=<optimized out>, system=..., parameters=..., argc=@0x7fffffffd83c: 1, argv=0x7fffffffd978)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/plugins/platforms/xcb/qxcbmain.cpp:50
#20 0x00007ffff47d9c3a in loadIntegration (argv=0x7fffffffd978, argc=@0x7fffffffd83c: 1, parameters=..., key=..., 
    loader=0x7ffff4c89c80 <_ZZN12_GLOBAL__N_112Q_QGS_loader13innerFunctionEvE6holder>)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/gui/kernel/qplatformintegrationfactory.cpp:56
#21 QPlatformIntegrationFactory::create (platform=..., paramList=..., argc=@0x7fffffffd83c: 1, argv=argv@entry=0x7fffffffd978, platformPluginPath=...)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/gui/kernel/qplatformintegrationfactory.cpp:73
#22 0x00007ffff47ea23c in init_platform (argv=0x7fffffffd978, argc=@0x7fffffffd83c: 1, platformThemeName=..., platformPluginPath=..., pluginArgument=...)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/gui/kernel/qguiapplication.cpp:1019
#23 QGuiApplicationPrivate::createPlatformIntegration (this=0x5555557772b0) at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/gui/kernel/qguiapplication.cpp:1176
#24 0x00007ffff47eaec5 in QGuiApplicationPrivate::createEventDispatcher (this=<optimized out>)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/gui/kernel/qguiapplication.cpp:1193
#25 0x00007ffff4499c58 in QCoreApplication::init (this=this@entry=0x7fffffffd840)
    at /var/tmp/portage/dev-qt/qtcore-5.5.9999/work/qtcore-5.5.9999/src/corelib/kernel/qcoreapplication.cpp:766
#26 0x00007ffff4499d09 in QCoreApplication::QCoreApplication (this=0x7fffffffd840, p=...)
    at /var/tmp/portage/dev-qt/qtcore-5.5.9999/work/qtcore-5.5.9999/src/corelib/kernel/qcoreapplication.cpp:688
#27 0x00007ffff47ed27f in QGuiApplication::QGuiApplication (this=0x7fffffffd840, p=...)
    at /var/tmp/portage/dev-qt/qtgui-5.5.9999/work/qtgui-5.5.9999/src/gui/kernel/qguiapplication.cpp:569
#28 0x00007ffff77e3d84 in QApplication::QApplication (this=0x7fffffffd840, argc=@0x7fffffffd83c: 1, argv=0x7fffffffd978, _internal=328960)
    at /var/tmp/portage/dev-qt/qtwidgets-5.5.9999/work/qtwidgets-5.5.9999/src/widgets/kernel/qapplication.cpp:569
#29 0x000055555555914b in main (argc=1, argv=0x7fffffffd978) at /home/jkt/work/prog/trojita/tests/Composer/test_Html_formatting.cpp:742