Description of problem:
After some timeout KDE session locked. Enter password. Panel freezes completely - no interaction of widgets update (just like static image).

(gdb) 
#0  0x00000038facf666d in poll () at /lib64/libc.so.6
#1  0x00000038c2e0a182 in _xcb_conn_wait () at /lib64/libxcb.so.1
#2  0x00000038c2e0b9f7 in wait_for_reply () at /lib64/libxcb.so.1
#3  0x00000038c2e0bb01 in xcb_wait_for_reply () at /lib64/libxcb.so.1
#4  0x00000038c32431f7 in _XReply () at /lib64/libX11.so.6
#5  0x00000038c664918a in DRI2GetBuffersWithFormat () at /lib64/libGL.so.1
#6  0x00000038c66494c7 in dri2GetBuffersWithFormat () at /lib64/libGL.so.1
#7  0x00007f2ccbb81dcb in intel_update_renderbuffers () at /usr/lib64/dri/i965_dri.so
#8  0x00007f2ccbb822e5 in intel_prepare_render () at /usr/lib64/dri/i965_dri.so
#9  0x00007f2ccbb76820 in brw_clear () at /usr/lib64/dri/i965_dri.so
#10 0x00007f2bf71bdab5 in Plotter::render() () at /usr/lib64/qt5/qml/org/kde/kquickcontrolsaddons/libkquickcontrolsaddonsplugin.so
#11 0x000000362bcaff07 in QMetaObject::activate(QObject*, int, int, void**) () at /lib64/libQt5Core.so.5
#12 0x00000038d2b908de in QQuickWindowPrivate::renderSceneGraph(QSize const&) () at /lib64/libQt5Quick.so.5
#13 0x00000038d2b6140c in QSGGuiThreadRenderLoop::renderWindow(QQuickWindow*) () at /lib64/libQt5Quick.so.5
#14 0x00000038d2b61f31 in QSGGuiThreadRenderLoop::event(QEvent*) () at /lib64/libQt5Quick.so.5
#15 0x00000038c97527ec in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib64/libQt5Widgets.so.5
#16 0x00000038c9757e90 in QApplication::notify(QObject*, QEvent*) () at /lib64/libQt5Widgets.so.5
#17 0x000000362bc7f67b in QCoreApplication::notifyInternal(QObject*, QEvent*) () at /lib64/libQt5Core.so.5
#18 0x000000362bcd598d in QTimerInfoList::activateTimers() () at /lib64/libQt5Core.so.5
#19 0x000000362bcd5e91 in timerSourceDispatch(_GSource*, int (*)(void*), void*) () at /lib64/libQt5Core.so.5
#20 0x00000038fcc49a8a in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
#21 0x00000038fcc49e20 in g_main_context_iterate.isra () at /lib64/libglib-2.0.so.0
#22 0x00000038fcc49ecc in g_main_context_iteration () at /lib64/libglib-2.0.so.0
#23 0x000000362bcd6b6f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib64/libQt5Core.so.5
#24 0x000000362bc7ceba in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib64/libQt5Core.so.5
#25 0x000000362bc84a2c in QCoreApplication::exec() () at /lib64/libQt5Core.so.5
#26 0x000000000042ebaf in main ()

Version-Release number of selected component (if applicable):
$ rpm -qa 'plasma*' '*kde*' '*xcb*' | sort
beefy-miracle-backgrounds-kde-16.91.0-7.fc21.noarch
beefy-miracle-kde-theme-16.91.0.3-2.fc21.noarch
f21-backgrounds-kde-21.1.0-1.fc22.noarch
f21-kde-theme-20.90-2.fc22.noarch
f22-backgrounds-kde-21.91.0-1.fc22.noarch
f22-kde-theme-22.3-4.fc22.noarch
flash-plugin-kde-11.2.202.460-1.fc22.R.x86_64
heisenbug-backgrounds-kde-20.0.0-2.fc21.noarch
heisenbug-kde-theme-19.90.5-3.fc22.noarch
kdeaccessibility-4.14.3-1.fc22.noarch
kdeartwork-common-15.04.0-1.fc22.noarch
kde-baseapps-15.04.2-1.fc22.x86_64
kde-baseapps-common-15.04.2-1.fc22.noarch
kde-baseapps-libs-15.04.2-1.fc22.x86_64
kde-cli-tools-5.3.1-2.fc22.x86_64
kdecoration-5.3.1-1.fc22.x86_64
kde-dev-scripts-15.04.0-1.fc22.noarch
kde-filesystem-4-51.fc22.x86_64
kdegames-4.14.3-1.fc22.noarch
kdegames-minimal-4.14.3-1.fc22.noarch
kdegraphics-libs-4.14.3-1.fc22.noarch
kdegraphics-strigi-analyzer-15.04.0-1.fc22.x86_64
kdegraphics-thumbnailers-15.04.0-1.fc22.x86_64
kde-i18n-3.5.10-20.fc21.noarch
kde-i18n-Russian-3.5.10-20.fc21.noarch
kde-l10n-14.12.3-6.fc22.noarch
kde-l10n-ru-14.12.3-6.fc22.noarch
kdelibs-4.14.9-2.fc22.x86_64
kdelibs-common-4.14.9-2.fc22.x86_64
kdelibs-ktexteditor-4.14.9-2.fc22.x86_64
kdelibs-webkit-4.14.9-2.fc22.x86_64
kdemultimedia-common-4.14.3-1.fc22.noarch
kdemultimedia-libs-4.14.3-1.fc22.noarch
kdenetwork-filesharing-15.04.0-1.fc22.x86_64
kdenetwork-strigi-analyzers-15.04.0-1.fc22.x86_64
kdeneur-0.17.0-2.fc21.x86_64
kdepasswd-15.04.2-1.fc22.x86_64  
kdepim-common-4.14.9-1.fc22.x86_64
kdepim-libs-4.14.9-1.fc22.x86_64
kdepimlibs-4.14.9-1.fc22.x86_64
kdepimlibs-akonadi-4.14.9-1.fc22.x86_64
kdepimlibs-gpgme-4.14.9-1.fc22.x86_64
kdepimlibs-kxmlrpcclient-4.14.9-1.fc22.x86_64
kdepim-runtime-4.14.9-1.fc22.x86_64
kdepim-runtime-libs-4.14.9-1.fc22.x86_64
kdeplasma-addons-5.3.1-1.fc22.x86_64
kde-plasma-qstardict-1.0.1-7.fc22.x86_64
kde-print-manager-15.03.95-2.fc22.x86_64
kde-print-manager-libs-15.03.95-2.fc22.x86_64
kde-runtime-15.04.2-1.fc22.x86_64
kde-runtime-drkonqi-15.04.2-1.fc22.x86_64
kde-runtime-flags-15.04.2-1.fc22.noarch
kde-runtime-libs-15.04.2-1.fc22.x86_64
kdesdk-common-4.14.3-1.fc22.noarch
kde-settings-22-10.fc22.noarch
kde-settings-kdm-22-10.fc22.noarch
kde-settings-ksplash-22-10.fc22.noarch
kde-settings-plasma-22-10.fc22.noarch
kde-settings-pulseaudio-22-10.fc22.noarch
kde-style-breeze-5.3.1-1.fc22.x86_64
kdesu-5.3.1-2.fc22.x86_64
kdeutils-4.14.3-1.fc22.noarch
kdeutils-common-4.14.3-1.fc22.noarch
kdeutils-minimal-4.14.3-1.fc22.noarch
kde-wallpapers-15.04.0-1.fc22.noarch
kde-workspace-common-4.11.20-2.fc22.noarch
kf5-kdeclarative-5.11.0-1.fc22.x86_64
kf5-kded-5.11.0-1.fc22.x86_64
kf5-kdelibs4support-5.11.0-1.fc22.x86_64
kf5-kdelibs4support-libs-5.11.0-1.fc22.x86_64
kf5-kdesu-5.11.0-1.fc22.x86_64
kf5-kdewebkit-5.11.0-1.fc22.x86_64
libkdegames-15.04.0-4.fc22.x86_64
libkdegames4-14.12.3-10.fc22.x86_64
libreoffice-kde-4.4.3.2-6.fc22.x86_64
libxcb-1.11-5.fc22.i686
libxcb-1.11-6.fc22.x86_64
libxcb-devel-1.11-6.fc22.x86_64
lockdev-1.0.4-0.19.20111007git.fc22.i686
lockdev-1.0.4-0.19.20111007git.fc22.x86_64
plasma-breeze-5.3.1-1.fc22.x86_64
plasma-breeze-common-5.3.1-1.fc22.noarch
plasma-desktop-5.3.1-3.fc22.x86_64
plasma-milou-5.3.1-1.fc22.x86_64
plasma-nm-5.3.1-1.fc22.x86_64
plasma-nm-openvpn-5.3.1-1.fc22.x86_64
plasma-nm-pptp-5.3.1-1.fc22.x86_64
plasma-nm-vpnc-5.3.1-1.fc22.x86_64
plasma-systemsettings-5.3.1-1.fc22.x86_64
plasma-workspace-5.3.1-2.fc22.x86_64
polkit-kde-5.3.1-1.fc22.x86_64
pykde4-4.14.3-5.fc22.x86_64
schroedinger-cat-backgrounds-kde-18.91.0-3.fc21.noarch
schroedinger-cat-kde-theme-18.91.6-3.fc21.noarch
sflphone-kde-video-1.4.1-3.fc21.x86_64
spherical-cow-backgrounds-kde-18.0.0-4.fc21.noarch
spherical-cow-kde-theme-18.0.3-2.fc21.noarch
verne-backgrounds-kde-15.92.1-8.fc21.noarch
verne-kde-theme-15.91.1-2.fc21.noarch
xcb-util-0.4.0-2.fc22.x86_64
xcb-util-cursor-0.1.1-5.fc22.x86_64
xcb-util-image-0.4.0-2.fc22.x86_64
xcb-util-keysyms-0.4.0-1.fc22.x86_64
xcb-util-renderutil-0.3.9-3.fc22.x86_64
xcb-util-wm-0.4.1-5.fc22.x86_64

How reproducible:
Frequently, but not always.