MDI example:

Thread 2 (Thread 0x7fffefd4e700 (LWP 27206)):
#0  0x00007ffff656c951 in __GI___poll (fds=0x7fffefd4dd38, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
        resultvar = 18446744073709551100
        sc_cancel_oldtype = 0
#1  0x00007ffff321d747 in ?? () from /usr/lib/x86_64-linux-gnu/libxcb.so.1
No symbol table info available.
#2  0x00007ffff321f53a in xcb_wait_for_event () from /usr/lib/x86_64-linux-gnu/libxcb.so.1
No symbol table info available.
#3  0x00007ffff7e8d469 in QXcbEventReader::run (this=0x555555587820) at qxcbconnection.cpp:1384
        event = <optimized out>
#4  0x00007ffff6c7f231 in QThreadPrivate::start (arg=0x555555587820) at thread/qthread_unix.cpp:367
        thr = 0x555555587820
        data = <optimized out>
        eventDispatcher = <optimized out>
        __clframe = {__cancel_routine = 0x7ffff6c7e3f0 <QThreadPrivate::finish(void*)>, __cancel_arg = 0x555555587820, __do_it = 1, __cancel_type = <optimized out>}
#5  0x00007ffff624c7fc in start_thread (arg=0x7fffefd4e700) at pthread_create.c:465
        pd = 0x7fffefd4e700
        now = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {140737217095424, -3173862719641307153, 140737488343166, 140737488343167, 140737217095424, 140737217096128, 3173897567902414831, 3173842454649288687}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x0, 0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 0}}}
        not_first_call = <optimized out>
        pagesize_m1 = <optimized out>
        sp = <optimized out>
        freesize = <optimized out>
        __PRETTY_FUNCTION__ = "start_thread"
#6  0x00007ffff6578b5f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
No locals.

Thread 1 (Thread 0x7ffff7fb3780 (LWP 27202)):
#0  __strncmp_sse42 () at ../sysdeps/x86_64/multiarch/strcmp-sse42.S:199
No locals.
#1  0x00007ffff7eb9821 in find_atom_pointer (table=table@entry=0x555555599a30, string=string@entry=0x55555560a7f0 "Shift", len=len@entry=5, atomp_out=atomp_out@entry=0x7fffffffcd20, fingerprint_out=fingerprint_out@entry=0x7fffffffcd1c) at ../../../3rdparty/xkbcommon/src/atom.c:151
        cmp = <optimized out>
        node = 0x5555555aba78
        atomp = 0x555555599a30
        fingerprint = <optimized out>
        found = false
#2  0x00007ffff7eb9a2e in atom_intern (table=0x555555599a30, string=0x55555560a7f0 "Shift", len=len@entry=5, steal=steal@entry=false) at ../../../3rdparty/xkbcommon/src/atom.c:202
        atomp = 0x2
        node = <optimized out>
        fingerprint = 21845
#3  0x00007ffff7ebdd3b in xkb_atom_intern (ctx=ctx@entry=0x5555555aac90, string=<optimized out>, len=len@entry=5) at ../../../3rdparty/xkbcommon/src/context-priv.c:61
No locals.
#4  0x00007ffff7ed127a in adopt_atoms (ctx=0x5555555aac90, conn=conn@entry=0x555555589330, from=from@entry=0x7fffe8001cd0, to=0x5555555c3650, count=count@entry=2) at ../../../3rdparty/xkbcommon/src/x11/util.c:186
        reply = 0x55555560a7d0
        i = 1
        start = 0
        stop = 2
        batch = 0
        cookies = {{sequence = 222}, {sequence = 223}, {sequence = 1431868208}, {sequence = 21845}, {sequence = 218}, {sequence = 0}, {sequence = 4294954752}, {sequence = 32767}, {sequence = 0}, {sequence = 0}, {sequence = 4294954720}, {sequence = 32767}, {sequence = 0}, {sequence = 0}, {sequence = 4079081810}, {sequence = 32767}, {sequence = 214}, {sequence = 0}, {sequence = 4294954800}, {sequence = 32767}, {sequence = 0}, {sequence = 0}, {sequence = 1969955840}, {sequence = 3573421483}, {sequence = 0}, {sequence = 0}, {sequence = 218}, {sequence = 0}, {sequence = 1431868208}, {sequence = 21845}, {sequence = 4294954712}, {sequence = 32767}, {sequence = 3892317264}, {sequence = 32767}, {sequence = 4294954720}, {sequence = 32767}, {sequence = 4294954712}, {sequence = 32767}, {sequence = 4132385870}, {sequence = 32767}, {sequence = 1431868208}, {sequence = 21845}, {sequence = 4079083101}, {sequence = 32767}, {sequence = 32}, {sequence = 0}, {sequence = 0}, {sequence = 0}, {sequence = 1431868208}, {sequence = 21845}, {sequence = 1969955840}, {sequence = 3573421483}, {sequence = 218}, {sequence = 0}, {sequence = 218}, {sequence = 0}, {sequence = 1431868208}, {sequence = 21845}, {sequence = 1969955840}, {sequence = 3573421483}, {sequence = 3892317264}, {sequence = 32767}, {sequence = 1431868208}, {sequence = 21845}, {sequence = 218}, {sequence = 0}, {sequence = 4294954752}, {sequence = 32767}, {sequence = 0}, {sequence = 0}, {sequence = 4294954720}, {sequence = 32767}, {sequence = 4294954712}, {sequence = 32767}, {sequence = 4079087953}, {sequence = 32767}, {sequence = 218}, {sequence = 0}, {sequence = 4079088277}, {sequence = 32767}, {sequence = 0}, {sequence = 0}, {sequence = 1432397776}, {sequence = 21845}, {sequence = 1431868232}, {sequence = 21845}, {sequence = 1432106752}, {sequence = 21845}, {sequence = 4129622447}, {sequence = 32767}, {sequence = 1431868208}, {sequence = 21845}, {sequence = 3}, {sequence = 0}, {sequence = 1969955840}, {sequence = 3573421483}, {sequence = 0}, {sequence = 0}, {sequence = 0}, {sequence = 0}, {sequence = 1432009288}, {sequence = 21845}, {sequence = 1432397776}, {sequence = 21845}, {sequence = 8}, {sequence = 0}, {sequence = 0}, {sequence = 0}, {sequence = 1431868208}, {sequence = 21845}, {sequence = 4132385870}, {sequence = 32767}, {sequence = 0}, {sequence = 0}, {sequence = 218}, {sequence = 0}, {sequence = 1431868232}, {sequence = 21845}, {sequence = 0}, {sequence = 0}, {sequence = 8}, {sequence = 0}, {sequence = 1969955840}, {sequence = 3573421483}, {sequence = 1432009800}, {sequence = 21845}, {sequence = 2}, {sequence = 0}}
#5  0x00007ffff7ed2957 in get_type_names (list=0x7fffffffd0b0, reply=0x7fffe8001c10, conn=0x555555589330, keymap=0x5555555ab9b0) at ../../../3rdparty/xkbcommon/src/x11/x11-keymap.c:867
        wire_type_name = <optimized out>
        wire_num_levels = 2 '\002'
        type = <optimized out>
        i = 1
        n_levels_per_type_length = <optimized out>
        n_levels_per_type_iter = 0x7fffe8001cb1 '\002' <repeats 11 times>, "\005\b\003\b\b\b\b\b\004\004\004\004\004\004\005\004\246"
        key_type_names_length = <optimized out>
        key_type_names_iter = 0x7fffe8001c44
        kt_level_names_iter = 0x7fffe8001cd0
#6  get_names (device_id=3, conn=0x555555589330, keymap=0x5555555ab9b0) at ../../../3rdparty/xkbcommon/src/x11/x11-keymap.c:1079
        cookie = <optimized out>
        reply = 0x7fffe8001c10
        list = {keycodesName = 163, geometryName = 32767, symbolsName = 217, physSymbolsName = 2000, typesName = 164, compatName = 164, typeNames = 0x7fffe8001c40, nLevelsPerType = 0x7fffe8001cb0 "\001", '\002' <repeats 11 times>, "\005\b\003\b\b\b\b\b\004\004\004\004\004\004\005\004\246", alignment_pad = 0x7fffe8001ccc "\246", ktLevelNames = 0x7fffe8001ccc, indicatorNames = 0x7fffe8001e8c, virtualModNames = 0x7fffe8001ec4, groups = 0x7fffe8001ef8, keyNames = 0x7fffe8001efc, keyAliases = 0x7fffe80022dc, radioGroupNames = 0x5555555aac90}
#7  xkb_x11_keymap_new_from_device (ctx=<optimized out>, conn=<optimized out>, device_id=3, flags=flags@entry=XKB_KEYMAP_COMPILE_NO_FLAGS) at ../../../3rdparty/xkbcommon/src/x11/x11-keymap.c:1154
        keymap = <optimized out>
        __func__ = "xkb_x11_keymap_new_from_device"
#8  0x00007ffff7e989e8 in QXcbKeyboard::updateKeymap (this=this@entry=0x5555555a9c90) at qxcbkeyboard.cpp:753
        keysymMods = {d = 0x7ffff6fd51c0 <QMapDataBase::shared_null>}
#9  0x00007ffff7e98ddc in QXcbKeyboard::QXcbKeyboard (this=0x5555555a9c90, connection=<optimized out>) at qxcbkeyboard.cpp:1179
No locals.
#10 0x00007ffff7e91371 in QXcbConnection::QXcbConnection (this=0x555555584410, nativeInterface=<optimized out>, canGrabServer=<optimized out>, defaultVisualId=<optimized out>, displayName=<optimized out>) at qxcbconnection.cpp:603
        dpy = <optimized out>
        extensions = {0x7ffff7fb18a0 <xcb_shm_id>, 0x7ffff7fb1880 <xcb_xfixes_id>, 0x7ffff7fb1890 <xcb_randr_id>, 0x7ffff7fb18d0 <xcb_shape_id>, 0x7ffff7fb18b0 <xcb_sync_id>, 0x7ffff7fb18e0 <xcb_xkb_id>, 0x7ffff7fb18c0 <xcb_render_id>, 0x0}
        glIntegrationNames = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x380}, d = 0x380}}, <No data fields>}
        glIntegrationName = {static null = {<No data fields>}, d = 0x40}
#11 0x00007ffff7e94b3e in QXcbIntegration::QXcbIntegration (this=<optimized out>, parameters=..., argc=@0x7fffffffd7ac: 1, argv=<optimized out>) at qxcbintegration.cpp:191
        displayName = <optimized out>
        noGrabArg = <optimized out>
        doGrabArg = <optimized out>
        conn = 0x7ffff7fbcf00
        canNotGrabEnv = false
#12 0x00007ffff7ff246b in QXcbIntegrationPlugin::create (this=<optimized out>, system=..., parameters=..., argc=@0x7fffffffd7ac: 1, argv=0x7fffffffd9f8) at qxcbmain.cpp:56
No locals.
#13 0x00007ffff7240e3d in qLoadPlugin<QPlatformIntegration, QPlatformIntegrationPlugin, QStringList const&, int&, char**&> (key=..., loader=<optimized out>) at ../../include/QtCore/5.11.0/QtCore/private/../../../../../src/corelib/plugin/qfactoryloader_p.h:114
        factoryObject = <optimized out>
        index = <optimized out>
#14 QPlatformIntegrationFactory::create (platform=..., paramList=..., argc=@0x7fffffffd7ac: 1, argv=argv@entry=0x7fffffffd9f8, platformPluginPath=...) at kernel/qplatformintegrationfactory.cpp:71
No locals.
#15 0x00007ffff725302c in init_platform (argv=<optimized out>, argc=@0x7fffffffd7ac: 1, platformThemeName=..., platformPluginPath=..., pluginNamesWithArguments=...) at kernel/qguiapplication.cpp:1159
        arguments = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x555555580640}, d = 0x555555580640}}, <No data fields>}
        name = {static null = {<No data fields>}, d = 0x55555557d5d0}
        argumentsKey = {static null = {<No data fields>}, d = 0x555555589060}
        pluginArgument = {static null = {<No data fields>}, d = 0x55555557d5d0}
        __for_range = @0x7fffffffd548: {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x55555557d600}, d = 0x55555557d600}}, <No data fields>}
        plugins = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x55555557d600}, d = 0x55555557d600}}, <No data fields>}
        platformArguments = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x7ffff6f88180 <QListData::shared_null>}, d = 0x7ffff6f88180 <QListData::shared_null>}}, <No data fields>}
        availablePlugins = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x555555581470}, d = 0x555555581470}}, <No data fields>}
        themeNames = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x555555589060}, d = 0x555555589060}}, <No data fields>}
#16 QGuiApplicationPrivate::createPlatformIntegration (this=0x55555557b940) at kernel/qguiapplication.cpp:1340
        platformPluginPath = {static null = {<No data fields>}, d = 0x7ffff6f86620 <QArrayData::shared_null>}
        platformName = {d = 0x55555557d5a0}
        platformNameEnv = {d = 0x7ffff6f86620 <QArrayData::shared_null>}
        platformThemeName = {static null = {<No data fields>}, d = 0x7ffff6f86620 <QArrayData::shared_null>}
        icon = {static null = {<No data fields>}, d = 0x7ffff6f86620 <QArrayData::shared_null>}
        j = <optimized out>
#17 0x00007ffff7253e7d in QGuiApplicationPrivate::createEventDispatcher (this=<optimized out>) at kernel/qguiapplication.cpp:1357
No locals.
#18 0x00007ffff6e5ba15 in QCoreApplicationPrivate::init (this=this@entry=0x55555557b940) at kernel/qcoreapplication.cpp:845
No locals.
#19 0x00007ffff7254bef in QGuiApplicationPrivate::init (this=this@entry=0x55555557b940) at kernel/qguiapplication.cpp:1386
        loadTestability = <optimized out>
        pluginList = {<QListSpecialMethods<QByteArray>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x0}, d = 0x0}}
        session_id = {static null = {<No data fields>}, d = 0x0}
        session_key = {static null = {<No data fields>}, d = 0xffffffffffffffff}
        s = {static null = {<No data fields>}, d = 0xffffffffffffffff}
        j = <optimized out>
        envPlugins = {d = 0x400000004}
#20 0x00007ffff78d4f49 in QApplicationPrivate::init (this=0x55555557b940) at kernel/qapplication.cpp:562
No locals.
#21 0x000055555555b127 in main (argc=<optimized out>, argv=0x7fffffffd9f8) at main.cpp:61
        app = {<QGuiApplication> = {<QCoreApplication> = {<QObject> = {_vptr.QObject = 0x7ffff7da4750 <vtable for QApplication+16>, static staticMetaObject = {d = {superdata = 0x0, stringdata = 0x7ffff7045760 <qt_meta_stringdata_QObject>, data = 0x7ffff7045640 <qt_meta_data_QObject>, static_metacall = 0x7ffff6e8b330 <QObject::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, d_ptr = {d = 0x55555557b940}, static staticQtMetaObject = {d = {superdata = 0x0, stringdata = 0x7ffff7048520 <qt_meta_stringdata_Qt>, data = 0x7ffff7045880 <qt_meta_data_Qt>, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}}, static staticMetaObject = {d = {superdata = 0x7ffff713e8c0 <QObject::staticMetaObject>, stringdata = 0x7ffff7041ca0 <qt_meta_stringdata_QCoreApplication>, data = 0x7ffff7041b80 <qt_meta_data_QCoreApplication>, static_metacall = 0x7ffff6e56300 <QCoreApplication::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, static self = 0x7fffffffd7d0}, static staticMetaObject = {d = {superdata = 0x7ffff7146600 <QCoreApplication::staticMetaObject>, stringdata = 0x7ffff766f680 <qt_meta_stringdata_QGuiApplication>, data = 0x7ffff766f400 <qt_meta_data_QGuiApplication>, static_metacall = 0x7ffff725b150 <QGuiApplication::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}}, static staticMetaObject = {d = {superdata = 0x7ffff77450c0 <QGuiApplication::staticMetaObject>, stringdata = 0x7ffff7c575e0 <qt_meta_stringdata_QApplication>, data = 0x7ffff7c57460 <qt_meta_data_QApplication>, static_metacall = 0x7ffff78d50c0 <QApplication::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}}
        parser = {d = 0x410}
        mainWin = {<QMainWindow> = {<QWidget> = {<QObject> = {_vptr.QObject = 0x2, static staticMetaObject = {d = {superdata = 0x0, stringdata = 0x7ffff7045760 <qt_meta_stringdata_QObject>, data = 0x7ffff7045640 <qt_meta_data_QObject>, static_metacall = 0x7ffff6e8b330 <QObject::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, d_ptr = {d = 0x0}, static staticQtMetaObject = {d = {superdata = 0x0, stringdata = 0x7ffff7048520 <qt_meta_stringdata_Qt>, data = 0x7ffff7045880 <qt_meta_data_Qt>, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}}, <QPaintDevice> = {_vptr.QPaintDevice = 0x6e0000005d, painters = 0, reserved = 0x0}, static staticMetaObject = {d = {superdata = 0x7ffff713e8c0 <QObject::staticMetaObject>, stringdata = 0x7ffff7c5a5e0 <qt_meta_stringdata_QWidget>, data = 0x7ffff7c59f00 <qt_meta_data_QWidget>, static_metacall = 0x7ffff790dcb0 <QWidget::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, data = 0x7c00000077}, static staticMetaObject = {d = {superdata = 0x7ffff7da5dc0 <QWidget::staticMetaObject>, stringdata = 0x7ffff7c73c00 <qt_meta_stringdata_QMainWindow>, data = 0x7ffff7c73a20 <qt_meta_data_QMainWindow>, static_metacall = 0x7ffff7a22050 <QMainWindow::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x7ffff7dafe90 <qt_meta_extradata_QMainWindow>, extradata = 0x0}}}, static staticMetaObject = {d = {superdata = 0x7ffff7dafe60 <QMainWindow::staticMetaObject>, stringdata = 0x555555564980 <qt_meta_stringdata_MainWindow>, data = 0x5555555647e0 <qt_meta_data_MainWindow>, static_metacall = 0x555555561ae0 <MainWindow::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, mdiArea = 0x4, windowMenu = 0x7ffff683ec20 <main_arena>, newAct = 0x7ffff683fd40 <initial>, saveAct = 0x410, saveAsAct = 0xeaf0, recentFileActs = {0x55555557b510, 0x1, 0x7ffff64f482a <__libc_calloc+746>, 0x7ffff683e6f8 <__exit_funcs>, 0x7ffff683fd40 <initial>}, recentFileSeparator = 0x555555568019 <(anonymous namespace)::dummy>, recentFileSubMenuAct = 0x7fffffffda08, cutAct = 0x7fffffffd9f8, copyAct = 0x7ffff64a00c0 <__new_exitfn+336>, pasteAct = 0x7fffffffda08, closeAct = 0x5555555619f0 <(anonymous namespace)::initializer::~initializer()>, closeAllAct = 0x555555568008, tileAct = 0x7ffff64a0169 <__GI___cxa_atexit+25>, cascadeAct = 0x1, nextAct = 0x2, previousAct = 0x5555555672c8, windowMenuSeparatorAct = 0x555555561f4d <__libc_csu_init+77>}