A similar issue arose for me after a recent update. The display freezes whenever I plug in my 4K external display. It happens in both gdm and Gnome, but not in X11, so my workaround for now is to disconnect the display in gdm and login to X11 before reconnecting the display.
-- System Information: Debian Release: buster/sid APT prefers testing APT policy: (500, 'testing'), (500, 'stable') Architecture: amd64 (x86_64) Foreign Architectures: i386 Kernel: Linux 4.15.0-2-amd64 (SMP w/4 CPU cores) Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US.$ Shell: /bin/sh linked to /bin/dash Init: systemd (via /run/systemd/system) LSM: AppArmor: enabled Versions of packages gnome-shell depends on: ii dconf-gsettings-backend [gsettings-backend] 0.26.1-3 ii evolution-data-server 3.26.5-2 ii gir1.2-accountsservice-1.0 0.6.45-1 ii gir1.2-atspi-2.0 2.28.0-1 ii gir1.2-freedesktop 1.56.0-2 ii gir1.2-gcr-3 3.28.0-1 ii gir1.2-gdesktopenums-3.0 3.28.0-1 ii gir1.2-gdm-1.0 3.28.0-1 ii gir1.2-geoclue-2.0 2.4.7-1 ii gir1.2-glib-2.0 1.56.0-2 ii gir1.2-gnomebluetooth-1.0 3.28.0-2 ii gir1.2-gnomedesktop-3.0 3.28.0-1 ii gir1.2-gtk-3.0 3.22.29-2 ii gir1.2-gweather-3.0 3.28.1-1 ii gir1.2-ibus-1.0 1.5.17-3 ii gir1.2-mutter-2 3.28.0-2 ii gir1.2-nm-1.0 1.10.6-2 ii gir1.2-nma-1.0 1.8.10-2 ii gir1.2-pango-1.0 1.40.14-1 ii gir1.2-polkit-1.0 0.105-18 ii gir1.2-rsvg-2.0 2.40.20-2 ii gir1.2-soup-2.4 2.62.0-1 ii gir1.2-upowerglib-1.0 0.99.7-2 ii gjs 1.52.0-2 ii gnome-backgrounds 3.28.0-1 ii gnome-settings-daemon 3.28.0-1 ii gnome-shell-common 3.28.0-1 ii gsettings-desktop-schemas 3.28.0-1 ii libatk-bridge2.0-0 2.26.2-1 ii libatk1.0-0 2.28.1-1 ii libc6 2.27-2 ii libcairo2 1.15.10-1 ii libcanberra-gtk3-0 0.30-6 ii libcanberra0 0.30-6 ii libcroco3 0.6.12-2 ii libecal-1.2-19 3.26.5-2 ii libedataserver-1.2-22 3.26.5-2 ii libgcr-base-3-1 3.28.0-1 ii libgdk-pixbuf2.0-0 2.36.11-2 ii libgirepository-1.0-1 1.56.0-2 ii libgjs0g [libgjs0-libmozjs-52-0] 1.52.0-2 ii libglib2.0-0 2.56.0-4 ii libglib2.0-bin 2.56.0-4 ii libgstreamer1.0-0 1.14.0-1 ii libgtk-3-0 3.22.29-2 ii libical3 3.0.1-5 ii libjson-glib-1.0-0 1.4.2-3 ii libmutter-2-0 3.28.0-2 ii libnm0 1.10.6-2 ii libpango-1.0-0 1.40.14-1 ii libpangocairo-1.0-0 1.40.14-1 ii libpolkit-agent-1-0 0.105-18 ii libpolkit-gobject-1-0 0.105-18 ii libpulse-mainloop-glib0 11.1-4 ii libpulse0 11.1-4 ii libsecret-1-0 0.18.5-6 ii libstartup-notification0 0.12-5 ii libsystemd0 238-3 ii libx11-6 2:1.6.5-1 ii libxfixes3 1:5.0.3-1 ii mutter 3.28.0-2 ii python3 3.6.4-1 Versions of packages gnome-shell recommends: ii chrome-gnome-shell 9-1 ii gdm3 3.28.0-1 ii gkbd-capplet 3.26.0-3 ii gnome-control-center 1:3.28.0-1 ii gnome-user-docs 3.28.0-1 ii iio-sensor-proxy 2.4-2 ii switcheroo-control 1.2-1 ii unzip 6.0-21 Versions of packages gnome-shell suggests: ii gir1.2-telepathyglib-0.12 0.24.1-2 ii gir1.2-telepathylogger-0.2 0.8.2-3 -- no debconf information
