I am experiencing the glitching reported to KDE for multiple distros (including another Gentoo user) at https://bugs.kde.org/show_bug.cgi?id=350976. Another user there reports that OpenJDK-1.8 resolves the problem, but we don't have dev-java/icedtea:8 in the java overlay at this time.

Portage 2.2.20.1 (python 3.4.1-final-0, default/linux/amd64/13.0/desktop/kde/systemd, gcc-4.8.5, glibc-2.20-r2, 4.0.5-gentoo x86_64)
=================================================================
                         System Settings
=================================================================
System uname: Linux-4.0.5-gentoo-x86_64-Intel-R-_Core-TM-_i7-3840QM_CPU_@_2.80GHz-with-gentoo-2.2
KiB Mem:    32805220 total,  20148380 free
KiB Swap:          0 total,         0 free
Timestamp of repository gentoo: Fri, 25 Sep 2015 06:00:01 +0000
sh bash 4.3_p39
ld GNU ld (Gentoo 2.25.1 p1.1) 2.25.1
app-shells/bash:          4.3_p39::gentoo
dev-java/java-config:     2.2.0::gentoo
dev-lang/perl:            5.20.2::gentoo
dev-lang/python:          2.7.9-r1::gentoo, 3.4.1::gentoo
dev-util/cmake:           3.2.2::gentoo
dev-util/pkgconfig:       0.28-r2::gentoo
sys-apps/baselayout:      2.2::gentoo
sys-apps/openrc:          0.17::gentoo
sys-apps/sandbox:         2.6-r1::gentoo
sys-devel/autoconf:       2.13::gentoo, 2.69::gentoo
sys-devel/automake:       1.11.6-r1::gentoo, 1.14.1::gentoo, 1.15::gentoo
sys-devel/binutils:       2.25.1-r1::gentoo
sys-devel/gcc:            4.8.5::gentoo
sys-devel/gcc-config:     1.7.3::gentoo
sys-devel/libtool:        2.4.6::gentoo
sys-devel/make:           4.1-r1::gentoo
sys-kernel/linux-headers: 3.18::gentoo (virtual/os-headers)
sys-libs/glibc:           2.20-r2::gentoo
Repositories:

gentoo
    location: /usr/portage
    sync-type: rsync
    sync-uri: rsync://rsync.gentoo.org/gentoo-portage
    priority: -1000
    sync-uri: git://github.com/pantera31752/open-overlay.git
    masters: gentoo
    priority: 50

steam-overlay
    location: /var/lib/layman/steam-overlay
    sync-type: laymansync
    sync-uri: git://github.com/anyc/steam-overlay.git
    masters: gentoo
    priority: 50

ACCEPT_KEYWORDS="amd64"
ACCEPT_LICENSE="*"
CBUILD="x86_64-pc-linux-gnu"
CFLAGS="-march=native -O2 -pipe -fomit-frame-pointer"
CHOST="x86_64-pc-linux-gnu"
CONFIG_PROTECT="/etc /usr/share/config /usr/share/gnupg/qualified.txt /usr/share/themes/oxygen-gtk/gtk-2.0 /var/lib/hsqldb"
CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/dconf /etc/env.d /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo"
CXXFLAGS="-march=native -O2 -pipe -fomit-frame-pointer"
DISTDIR="/usr/portage/distfiles"
EMERGE_DEFAULT_OPTS=" --with-bdeps y --binpkg-respect-use=y"
FCFLAGS="-O2 -pipe"
FEATURES="assume-digests binpkg-logs buildpkg collision-protect config-protect-if-modified distlocks ebuild-locks fixlafiles merge-sync news parallel-fetch preserve-libs protect-owned sandbox sfperms strict unknown-features-warn unmerge-logs unmerge-orphans userfetch userpriv usersandbox usersync xattr"
FFLAGS="-O2 -pipe"
GENTOO_MIRRORS="http://distfiles.gentoo.org"
LANG="en_US.UTF-8"
LC_ALL="en_US.UTF-8"
LDFLAGS="-Wl,-O1 -Wl,--as-needed,-z,now,--hash-style=gnu,--sort-common"
MAKEOPTS="-j9"
PKGDIR="/usr/portage/packages"
PORTAGE_CONFIGROOT="/"
PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --omit-dir-times --compress --force --whole-file --delete --stats --human-readable --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages"
PORTAGE_TMPDIR="/var/tmp"
USE="X a52 aac acl acpi aes alsa amd64 audiofile avahi avx bash-completion berkdb branding bzip2 cairo caps cdaudio cdda cddb cdio cdparanoia cdr cleartype cli cracklib crypt css cups curl cxx dbus declarative dri dts dv dvd dvdr dvdread efi emboss encode exif faad fam fame fastcgi fat fbsplash fdk ffmpeg fftw firefox flac fortran ftp gd gdbm geoip gif git glamor glib glut gmp gnutls gphoto2 gpm gtk hbci iconv icu idn imagemagick imap imlib ipv6 ithreads jack javascript jfs jpeg kde kdehiddenvisibility kipi lame laptop lcms libnotify lighttpd live lzo mad matroska mcal memlimit mjpeg mmap mmx mmxext mng modules mozilla mp3 mp4 mpeg multilib musepack musicbrainz ncurses network networkmanager nls nntp nptl nsplugin ntfs offensive ofx ogg openexr opengl opus pam pango pch pcre pdf phonon physfs plasma png policykit popcnt postgres ppds pulseaudio python python3 qt3support qt4 rackspace rdesktop readline real reiserfs resolvconf rtc schroedinger sdl seccomp semantic-desktop session slp speex spell sse sse2 sse3 sse4_1 sse4_2 ssl ssse3 startup-notification streaming svg symlink syslog systemd taglib theora threads threadsafe thunderbird tidy tiff tk truetype udev udisks unicode upnp upower usb utempter utf8 v4l vaapi vcd vhosts video vim-syntax vnc vorbis webdav win32codecs wps wxwidgets x264 xattr xcb xcomposite xinerama xml xosd xscreensaver xv xvid zlib" ABI_X86="64 32" ALSA_CARDS="ali5451 als4000 atiixp atiixp-modem bt87x ca0106 cmipci emu10k1x ens1370 ens1371 es1938 es1968 fm801 hda-intel intel8x0 intel8x0m maestro3 trident usb-audio via82xx via82xx-modem ymfpci" APACHE2_MODULES="alias auth_basic authn_alias authn_default authn_file authz_default authz_groupfile authz_host authz_user autoindex dav dav_fs dir env expires headers include info log_config mime mime_magic negotiation proxy proxy_http rewrite setenvif status unique_id" APACHE2_MPMS="event" CALLIGRA_FEATURES="kexi words flow plan sheets stage tables krita karbon braindump author" CAMERAS="directory" COLLECTD_PLUGINS="df interface irq load memory rrdtool swap syslog" CPU_FLAGS_X86="aes avx mmx mmxext popcnt sse sse2 sse3 sse4_1 sse4_2 ssse3" ELIBC="glibc" GPSD_PROTOCOLS="ashtech aivdm earthmate evermore fv18 garmin garmintxt gpsclock itrax mtk3301 nmea ntrip navcom oceanserver oldstyle oncore rtcm104v2 rtcm104v3 sirf superstar2 timing tsip tripmate tnt ublox ubx" INPUT_DEVICES="evdev synaptics" KERNEL="linux" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LIBREOFFICE_EXTENSIONS="presenter-console presenter-minimizer" LINGUAS="en en_US" NERONE_USERS="mike" OFFICE_IMPLEMENTATION="libreoffice" PHP_TARGETS="php5-5" PYTHON_SINGLE_TARGET="python2_7" PYTHON_TARGETS="python2_7 python3_4" QEMU_SOFTMMU_TARGETS="x86_64" QEMU_USER_TARGETS="x86_64" RUBY_TARGETS="ruby20 ruby21" USERLAND="GNU" VIDEO_CARDS="intel nvidia" XTABLES_ADDONS="quota2 psd pknock lscan length2 ipv4options ipset ipp2p iface geoip fuzzy condition tee tarpit sysrq steal rawnat logmark ipmark dhcpmac delude chaos account"
Unset:  CC, CPPFLAGS, CTARGET, CXX, INSTALL_MASK, PORTAGE_BUNZIP2_COMMAND, PORTAGE_COMPRESS, PORTAGE_COMPRESS_FLAGS, PORTAGE_RSYNC_EXTRA_OPTS, USE_PYTHON

=================================================================
                        Package Settings
=================================================================

dev-java/icedtea-7.2.6.1::gentoo was built with the following:
USE="alsa awt cups gtk javascript jbootstrap nsplugin pulseaudio sunec webstart -cacao -cjk -debug -doc -examples -infinality -jamvm -kerberos -nss -pax_kernel -sctp (-selinux) -smartcard -source -test -zero" ABI_X86="64"