After world update and rebooting with "gdm" set in /etc/conf.d/xdm, complete system hang. Could not switch to any TTY. Switched to lightdm in /etc/conf.d/xdm and startx works but system is really laggy and major graphics corruption issues with Gnome 3.36. Fonts are garbled and randomly change to black squares, any mouseover on buttons creates a black box and graphics tearing. title bars are skewed and menus do not show.  I have included the following information as other seem to not have this issue. This is on a Thinkpad T440p with integrated Intel graphics.

/etc/portage/make.conf is clean (nothing added)

~$ lspci -v | less

00:02.0 VGA compatible controller: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller (rev 06) (prog-if 00 [VGA controller])
Subsystem: Lenovo ThinkPad T440p
Flags: bus master, fast devsel, latency 0, IRQ 30
Memory at f0000000 (64-bit, non-prefetchable) [size=4M]
Memory at e0000000 (64-bit, prefetchable) [size=256M]
I/O ports at 3000 [size=64]
[virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
Capabilities: <access denied>
Kernel driver in use: i915
Kernel modules: i915

~ $ epro list

=== arch: ===

arm-32bit, arm-64bit, riscv-64bit, x86-32bit, (x86-64bit*), x86-64bit-papa

=== build: ===

(current*), musl

=== subarch: ===

amd64-bulldozer, amd64-excavator, amd64-jaguar, amd64-k10
amd64-k8, amd64-k8+sse3, amd64-piledriver, amd64-steamroller
amd64-zen, atom_64, btver1_64, core-avx-i, core2_64, corei7
generic_64, intel64-broadwell, intel64-haswell*, intel64-ivybridge
intel64-nehalem, intel64-sandybridge, intel64-silvermont, intel64-skylake
intel64-skylake-avx512, intel64-westmere, native_64, nocona
opteron_64, xen-pentium4+sse3_64

=== flavor: ===

core, desktop*, hardened, minimal, server, workstation

=== mix-ins: ===

X, amazon-ec2, audio, cinnamon, console-extras, dvd, encrypted-root
gfxcard-amdgpu, gfxcard-ancient-ati, gfxcard-intel*, gfxcard-nouveau
gfxcard-nvidia, gfxcard-nvidia-legacy, gfxcard-older-ati, gfxcard-radeon
gnome*, gnome-3.16-fixups, hardened, kde, kde-plasma-5, kde-plasma-5-new
lxde, lxqt, mate, media, media-pro, mediadevice-audio-consumer
mediadevice-audio-pro, mediadevice-base, mediadevice-video-consumer
mediadevice-video-pro, mediaformat-audio-common, mediaformat-audio-extra
mediaformat-gfx-common, mediaformat-gfx-extra, mediaformat-video-common
mediaformat-video-extra, no-emul-linux-x86, no-systemd, openvz-host
print, python3-only, selinux, stage1, vmware-guest, wayland
xfce

~$ eselect opengl list

Available OpenGL implementations:

[1] xorg-x11 *

~$ lsmod | grep i915

i915                      2490368 4
video                    53248 2 thinkpad_acpi,i915
drm_kms_helper  237568 1 i915
cec                       61440 2 drm_kms_helper,i915
drm                       593920 4 drm_kms_helper,i915
i2c_algo_bit          16384 1 i915