I was closing steam (takes a while to close) and also alt-tabbing and kwin crashed.

Tried reproducing it and could not.

Application: KWin (kwin), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7fbe937c7800 (LWP 2852))]

Thread 3 (Thread 0x7fbe74a05700 (LWP 27175)):
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
#1  0x00007fbe921aaffb in QTWTF::TCMalloc_PageHeap::scavengerThread (this=0x7fbe924a8f00 <QTWTF::pageheap_memory>) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/FastMalloc.cpp:2359
#2  0x00007fbe921ab039 in QTWTF::TCMalloc_PageHeap::runScavengerThread (context=<optimized out>) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/FastMalloc.cpp:1464
#3  0x00007fbe8ad50182 in start_thread (arg=0x7fbe74a05700) at pthread_create.c:312
#4  0x00007fbe92fbc30d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Thread 2 (Thread 0x7fbdee982700 (LWP 27176)):
#0  0x00007fbe92fb3c33 in select () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007fbe8d3a52c7 in qt_safe_select (nfds=13, fdread=0x7fbe70000ac8, fdwrite=0x7fbe70000d60, fdexcept=0x7fbe70000ff8, orig_timeout=0x0) at kernel/qcore_unix.cpp:79
#2  0x00007fbe8d3aa824 in QEventDispatcherUNIXPrivate::doSelect (this=this@entry=0x7fbe70000910, flags=..., timeout=0x0) at kernel/qeventdispatcher_unix.cpp:223
#3  0x00007fbe8d3aac92 in QEventDispatcherUNIX::processEvents (this=0x7fbe700008f0, flags=...) at kernel/qeventdispatcher_unix.cpp:936
#4  0x00007fbe8d3790af in QEventLoop::processEvents (this=this@entry=0x7fbdee981da0, flags=...) at kernel/qeventloop.cpp:149
#5  0x00007fbe8d3793a5 in QEventLoop::exec (this=this@entry=0x7fbdee981da0, flags=...) at kernel/qeventloop.cpp:204
#6  0x00007fbe8d275c5f in QThread::exec (this=this@entry=0x187a060) at thread/qthread.cpp:537
#7  0x00007fbe8d35a823 in QInotifyFileSystemWatcherEngine::run (this=0x187a060) at io/qfilesystemwatcher_inotify.cpp:265
#8  0x00007fbe8d27832f in QThreadPrivate::start (arg=0x187a060) at thread/qthread_unix.cpp:349
#9  0x00007fbe8ad50182 in start_thread (arg=0x7fbdee982700) at pthread_create.c:312
#10 0x00007fbe92fbc30d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Thread 1 (Thread 0x7fbe937c7800 (LWP 2852)):
[KCrash Handler]
#6  0x00000000018463e0 in ?? ()
#7  0x00007fbe932fe1ad in KWin::Workspace::constrainedStackingOrder (this=this@entry=0x17d9e50) at ../../kwin/layers.cpp:509
#8  0x00007fbe932fe780 in KWin::Workspace::updateStackingOrder (this=this@entry=0x17d9e50, propagate_new_clients=<optimized out>) at ../../kwin/layers.cpp:117
#9  0x00007fbe932feb1d in updateStackingOrder (propagate_new_clients=<optimized out>, this=0x17d9e50) at ../../kwin/layers.cpp:609
#10 KWin::Workspace::blockStackingUpdates (this=this@entry=0x17d9e50, block=block@entry=false) at ../../kwin/layers.cpp:616
#11 0x00007fbe932ee7eb in ~StackingUpdatesBlocker (this=<synthetic pointer>, __in_chrg=<optimized out>) at ../../kwin/workspace.h:565
#12 KWin::Client::destroyClient (this=this@entry=0x199e8a0) at ../../kwin/client.cpp:355
#13 0x00007fbe93309e71 in KWin::Client::unmapNotifyEvent (this=0x199e8a0, e=<optimized out>) at ../../kwin/events.cpp:597
#14 0x00007fbe9330bfeb in KWin::Client::windowEvent (this=0x199e8a0, e=e@entry=0x7fffc5325a90) at ../../kwin/events.cpp:451
#15 0x00007fbe9330d62f in KWin::Workspace::workspaceEvent (this=0x17d9e50, e=e@entry=0x7fffc5325a90) at ../../kwin/events.cpp:164
#16 0x00007fbe93301970 in KWin::Application::x11EventFilter (this=0x7fffc5325e80, e=0x7fffc5325a90) at ../../kwin/main.cpp:422
#17 0x00007fbe8c77c5dc in qt_x11EventFilter (ev=0x7fffc5325a90) at kernel/qapplication_x11.cpp:442
#18 0x00007fbe8c78ce40 in QApplication::x11ProcessEvent (this=0x7fffc5325e80, event=event@entry=0x7fffc5325a90) at kernel/qapplication_x11.cpp:3461
#19 0x00007fbe8c7b6470 in QEventDispatcherX11::processEvents (this=0x1602af0, flags=...) at kernel/qeventdispatcher_x11.cpp:132
#20 0x00007fbe8d3790af in QEventLoop::processEvents (this=this@entry=0x7fffc5325cf0, flags=...) at kernel/qeventloop.cpp:149
#21 0x00007fbe8d3793a5 in QEventLoop::exec (this=this@entry=0x7fffc5325cf0, flags=...) at kernel/qeventloop.cpp:204
#22 0x00007fbe8d37eb79 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1225
#23 0x00007fbe8c71237c in QApplication::exec () at kernel/qapplication.cpp:3828
#24 0x00007fbe93302856 in kdemain (argc=3, argv=0x7fffc5325fc8) at ../../kwin/main.cpp:597
#25 0x00007fbe92ee2ec5 in __libc_start_main (main=0x4006d0 <main(int, char**)>, argc=3, argv=0x7fffc5325fc8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffc5325fb8) at libc-start.c:287
#26 0x00000000004006fe in _start ()