(gdb) bt
#0  0x00007ffff341208d in nanosleep () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff343feb4 in usleep () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x0000000000471726 in main_window::playloop_step (this=0x8d0db0)
    at gui.cpp:3292
#3  0x00007ffff4a14281 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#4  0x00007ffff4a19179 in QObject::event(QEvent*) ()
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#5  0x00007ffff4f22894 in QApplicationPrivate::notify_helper(QObject*, QEvent*)
    () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#6  0x00007ffff4f27713 in QApplication::notify(QObject*, QEvent*) ()
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#7  0x00007ffff49ffe9c in QCoreApplication::notifyInternal(QObject*, QEvent*)
    () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#8  0x00007ffff4a311f2 in ?? () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#9  0x00007ffff4a2ec0d in ?? () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#10 0x00007ffff4a2ec31 in ?? () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#11 0x00007fffeec2ad13 in g_main_context_dispatch ()
   from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#12 0x00007fffeec2b060 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#13 0x00007fffeec2b124 in g_main_context_iteration ()
   from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#14 0x00007ffff4a2f3bf in QEventDispatcherGlib::processEvents(QFlags<QEventLoop: