Application: kwin_x11 (5.5.4)

Qt Version: 5.5.1
Operating System: Linux 4.2.0-30-generic x86_64
Distribution: Ubuntu 15.10

-- Information about the crash:
- What I was doing when the application crashed:

Rebooting the labtop or Logout then Login

- Unusual behavior I noticed:

There's a small black square stuck in the top left corner unable to be moved and always on top. When I ctrl-alt-esc kill it, a xembedsniproxy process vanishes

- Custom settings of the application:

This is an upgrade from previous versions of Kubuntu, and is using the Backport PPA

The crash can be reproduced every time.

-- Backtrace:
Application: KWin (kwin_x11), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f44e5a7c880 (LWP 4752))]

Thread 4 (Thread 0x7f44d6e6c700 (LWP 4764)):
#0  0x00007f44fdbea6f3 in select () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007f44fc1ccb7f in qt_safe_select(int, fd_set*, fd_set*, fd_set*, timespec const*) (nfds=15, fdread=fdread@entry=0x7f44c8000a98, fdwrite=fdwrite@entry=0x7f44c8000d28, fdexcept=fdexcept@entry=0x7f44c8000fb8, orig_timeout=orig_timeout@entry=0x0) at kernel/qcore_unix.cpp:75
#2  0x00007f44fc1ce67e in QEventDispatcherUNIXPrivate::doSelect(QFlags<QEventLoop::ProcessEventsFlag>, timespec*) (timeout=0x0, exceptfds=0x7f44c8000fb8, writefds=0x7f44c8000d28, readfds=0x7f44c8000a98, nfds=<optimized out>, this=0x7f44c80008e0) at kernel/qeventdispatcher_unix.cpp:320
#3  0x00007f44fc1ce67e in QEventDispatcherUNIXPrivate::doSelect(QFlags<QEventLoop::ProcessEventsFlag>, timespec*) (this=this@entry=0x7f44c8000900, flags=..., flags@entry=..., timeout=timeout@entry=0x0) at kernel/qeventdispatcher_unix.cpp:196
#4  0x00007f44fc1cebae in QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (this=0x7f44c80008e0, flags=...) at kernel/qeventdispatcher_unix.cpp:607
#5  0x00007f44fc17850a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (this=this@entry=0x7f44d6e6bd20, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#6  0x00007f44fbf94ac4 in QThread::exec() (this=<optimized out>) at thread/qthread.cpp:503
#7  0x00007f44f6ccbc35 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#8  0x00007f44fbf99a6e in QThreadPrivate::start(void*) (arg=0x1324c50) at thread/qthread_unix.cpp:331
#9  0x00007f44fd1f26aa in start_thread (arg=0x7f44d6e6c700) at pthread_create.c:333
#10 0x00007f44fdbf3e9d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

Thread 3 (Thread 0x7f44d5d82700 (LWP 4768)):
#0  0x00007f44fd1f7da0 in pthread_cond_wait@@GLIBC_2.3.2 () at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
#1  0x00007f44fb122194 in  () at /usr/lib/x86_64-linux-gnu/libQt5Script.so.5
#2  0x00007f44fb1221d9 in  () at /usr/lib/x86_64-linux-gnu/libQt5Script.so.5
#3  0x00007f44fd1f26aa in start_thread (arg=0x7f44d5d82700) at pthread_create.c:333
#4  0x00007f44fdbf3e9d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

Thread 2 (Thread 0x7f44d7bdf700 (LWP 5143)):
#0  0x00007f44fdbea6f3 in select () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007f44fc1ccb7f in qt_safe_select(int, fd_set*, fd_set*, fd_set*, timespec const*) (nfds=14, fdread=fdread@entry=0x7f44d000b9c8, fdwrite=fdwrite@entry=0x7f44d000bc58, fdexcept=fdexcept@entry=0x7f44d000bee8, orig_timeout=orig_timeout@entry=0x0) at kernel/qcore_unix.cpp:75
#2  0x00007f44fc1ce67e in QEventDispatcherUNIXPrivate::doSelect(QFlags<QEventLoop::ProcessEventsFlag>, timespec*) (timeout=0x0, exceptfds=0x7f44d000bee8, writefds=0x7f44d000bc58, readfds=0x7f44d000b9c8, nfds=<optimized out>, this=0x7f44d00faa80) at kernel/qeventdispatcher_unix.cpp:320
#3  0x00007f44fc1ce67e in QEventDispatcherUNIXPrivate::doSelect(QFlags<QEventLoop::ProcessEventsFlag>, timespec*) (this=this@entry=0x7f44d000b830, flags=..., flags@entry=..., timeout=timeout@entry=0x0) at kernel/qeventdispatcher_unix.cpp:196
#4  0x00007f44fc1cebae in QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (this=0x7f44d00faa80, flags=...) at kernel/qeventdispatcher_unix.cpp:607
#5  0x00007f44fc17850a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (this=this@entry=0x7f44d7bded20, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#6  0x00007f44fbf94ac4 in QThread::exec() (this=<optimized out>) at thread/qthread.cpp:503
#7  0x00007f44f6ccbc35 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#8  0x00007f44fbf99a6e in QThreadPrivate::start(void*) (arg=0x189e840) at thread/qthread_unix.cpp:331
#9  0x00007f44fd1f26aa in start_thread (arg=0x7f44d7bdf700) at pthread_create.c:333
#10 0x00007f44fdbf3e9d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

Thread 1 (Thread 0x7f44e5a7c880 (LWP 4752)):
[KCrash Handler]
#6  0x00007f44f6be4d35 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#7  0x00007f44f6b8bbd5 in QV4::ExecutionEngine::newStringObject(QV4::Value const&) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#8  0x00007f44f6c25092 in QV4::Runtime::getProperty(QV4::ExecutionEngine*, QV4::Value const&, int) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#9  0x00007f44d4235c79 in  ()
#10 0x00007f44f73f97a2 in QQuickItem::staticMetaObject () at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#11 0x00007f44d47c4260 in  ()
#12 0x00000000018741a0 in  ()
#13 0x00007f44d47c4258 in  ()
#14 0x0000000001c71a80 in  ()
#15 0x00007f44fc4e0119 in (anonymous namespace)::construct(QVariant::Private*, void const*) (this=<synthetic pointer>, tc=<synthetic pointer>) at ../../include/QtCore/5.5.1/QtCore/private/../../../../../src/corelib/kernel/qvariant_p.h:339
#16 0x00007f44fc4e0119 in (anonymous namespace)::construct(QVariant::Private*, void const*) (this=<optimized out>, tc=<synthetic pointer>) at ../../include/QtCore/5.5.1/QtCore/private/../../../../../src/corelib/kernel/qvariant_p.h:362
#17 0x00007f44fc4e0119 in (anonymous namespace)::construct(QVariant::Private*, void const*) (this=<synthetic pointer>) at ../../include/QtCore/5.5.1/QtCore/private/../../../../../src/corelib/kernel/qvariant_p.h:383
#18 0x00007f44fc4e0119 in (anonymous namespace)::construct(QVariant::Private*, void const*) (data=0x0, type=<optimized out>, logic=<synthetic pointer>) at ../../include/QtCore/5.5.1/QtCore/private/../../../../../src/corelib/kernel/qmetatypeswitcher_p.h:68
#19 0x00007f44fc4e0119 in (anonymous namespace)::construct(QVariant::Private*, void const*) (x=0x17f2d00, copy=0x7ffd044d5810) at kernel/qguivariant.cpp:101
#20 0x0000000001c71a80 in  ()
#21 0x0000000001c71a80 in  ()
#22 0x00000000017504d8 in  ()
#23 0x0000000000000003 in  ()
#24 0x00000000020c4470 in  ()
#25 0x00007f44f6e0de20 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#26 0x00007f44d47c41f8 in  ()
#27 0x0000000001c71a80 in  ()
#28 0x00007ffd044d5dd0 in  ()
#29 0x00007f44d47a35e0 in  ()
#30 0x0000000000000000 in  ()

Reported using DrKonqi