please install the debug packagex for kwin, re-login and re-cause the crash, then post the new backtrace. the present report doesn't provide the slightest hint what segfaulted :-(

see https://techbase.kde.org/Development/Tutorials/Debugging/How_to_create_useful_crash_reports for details

I have precisely the same problem.  For me it happens not always, but sometimes.  Here is my crash report:

Application: kwin (4.11.13)
KDE Platform Version: 4.14.2
Qt Version: 4.8.6
Operating System: Linux 3.16.0-4-686-pae i686
Distribution: Debian GNU/Linux 8.2 (jessie)

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

The following is not happening always, but already happened several times:

1. I finish an email with Thunderbird and click on "send".
2. While Thunderbird is still sending the email, I am pressing ALT+TAB to select another window.
3. While I am still wondering which window to select next, Thunderbird finishes sending the mail and closes the window.
4. At this moment, kwin crashes.

The crash can be reproduced sometimes.

-- Backtrace:
Application: KWin (kwin), signal: Segmentation fault
Using host libthread_db library "/lib/i386-linux-gnu/i686/cmov/libthread_db.so.1".
[Current thread is 1 (Thread 0xb0738780 (LWP 24013))]

Thread 2 (Thread 0xacd38b40 (LWP 24037)):
#0  0xb7735d40 in __kernel_vsyscall ()
#1  0xb47ddc4b in pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/i386/i686/../i486/pthread_cond_wait.S:187
#2  0xb74631bc in __pthread_cond_wait (cond=0xb6f608f0, mutex=0xb6f608d8) at forward.c:149
#3  0xb6e4744f in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#4  0xb6e4748c in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#5  0xb47d9efb in start_thread (arg=0xacd38b40) at pthread_create.c:309
#6  0xb745662e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:129

Thread 1 (Thread 0xb0738780 (LWP 24013)):
[KCrash Handler]
#7  0xb75136b8 in main_arena () from /lib/i386-linux-gnu/i686/cmov/libc.so.6
#8  0xb759ce97 in KWin::Workspace::constrainedStackingOrder (this=0x9efc680) at ../../kwin/layers.cpp:509
#9  0xb759d482 in KWin::Workspace::updateStackingOrder (this=this@entry=0x9efc680, propagate_new_clients=<optimized out>) at ../../kwin/layers.cpp:117
#10 0xb759d8bf in updateStackingOrder (propagate_new_clients=<optimized out>, this=0x9efc680) at ../../kwin/layers.cpp:615
#11 KWin::Workspace::blockStackingUpdates (this=0x9efc680, block=false) at ../../kwin/layers.cpp:616
#12 0xb758b89f in ~StackingUpdatesBlocker (this=<optimized out>, __in_chrg=<optimized out>) at ../../kwin/workspace.h:565
#13 KWin::Client::releaseWindow (this=0xa2d0598, on_shutdown=false) at ../../kwin/client.cpp:317
#14 0xb75ab01b in KWin::Client::unmapNotifyEvent (this=0xa2d0598, e=0xbfbb741c) at ../../kwin/events.cpp:595
#15 0xb75ad37a in KWin::Client::windowEvent (this=0xa2d0598, e=0xbfbb741c) at ../../kwin/events.cpp:451
#16 0xb75ae8a8 in KWin::Workspace::workspaceEvent (this=0x9efc680, e=0xbfbb741c) at ../../kwin/events.cpp:161
#17 0xb75a0f1a in KWin::Application::x11EventFilter (this=0xbfbb7684, e=0xbfbb741c) at ../../kwin/main.cpp:422
#18 0xb50a3d4c in qt_x11EventFilter (ev=0xbfbb741c) at kernel/qapplication_x11.cpp:436
#19 0xb50b35ff in QApplication::x11ProcessEvent (this=0xbfbb7684, event=0xbfbb741c) at kernel/qapplication_x11.cpp:3375
#20 0xb50dff51 in QEventDispatcherX11::processEvents (this=0x9dfbec0, flags=...) at kernel/qeventdispatcher_x11.cpp:132
#21 0xb5b89d9f in QEventLoop::processEvents (this=0xbfbb75a4, flags=...) at kernel/qeventloop.cpp:149
#22 0xb5b8a12e in QEventLoop::exec (this=0xbfbb75a4, flags=...) at kernel/qeventloop.cpp:204
#23 0xb5b902b6 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1227
#24 0xb5029614 in QApplication::exec () at kernel/qapplication.cpp:3828
#25 0xb75a2128 in kdemain (argc=3, argv=0xbfbb7784) at ../../kwin/main.cpp:597
#26 0x0804856b in main (argc=3, argv=0xbfbb7784) at kwin_dummy.cpp:3

Possible duplicates by query: bug 350814.

An https://bugs.kde.org/ berichten

Looks like bug #317765 - do you use compositing? (in doubt attach the output of "qdbus org.kde.KWin /KWin supportInformation")

It seems that compositing is off:

$ qdbus org.kde.KWin /KWin supportInformation | grep omposit
compositingMode: 1
useCompositing: false
compositingInitialized: false
Compositing
Compositing is not active

Fits the pattern, you encounter bug #317765

Mike, no compositing either (when getting this crash)?

Sorry for the long delay.  Also I have not been able to install the debugging kwin yet.

I am trying to use the same setup on two different laptops.

On the laptop with the crash I have the following results:

prompt> qdbus org.kde.KWin /KWin supportInformation | grep omposit
compositingMode: 1
useCompositing: true
compositingInitialized: false
Compositing
Compositing is not active

I'm not at the other laptop at the moment; I believe it differs by having "useCompositing" as false instead of true.

For some reason it has become more difficult for me to reproduce this bug.  I try the same Alt-Tab to leave a Thunderbird window that is sending the message, and kwin no longer reports that it is crashing.  I have not consciously done anything other than posting the original submission here.

Fits the pattern. Unless  backtrace shows different, this wiil be bug #317765

*** This bug has been marked as a duplicate of bug 317765 ***