Run nixpkgs' gnome shell inside a `xephyr` window

gnome-shell.nix

# NOTE: unfortunately `nixpkgs` xephyr doesn't nest properly in my testing.
# So you need to have a system Xephyr.
#
# This script will create a ./scratch directory in the current directory,
# where it'll store temporary files and a makeshift $HOME dir.
#
# To build the script:
#     nix-build --out-link gnome-shell.sh gnome-shell.nix
#
# Then run it with:
#     ./gnome-shell.sh

{pkgs ? import <nixpkgs> {}, display ? ":1", extraEnv ? "", xephyr ? "/usr/bin/Xephyr" }:
with pkgs;
let dbus-services = with gnome3; [
	dconf
	gnome_settings_daemon
]; in
writeScript "gnome-shell-wrapper" ''#!${bash}/bin/bash
	set -eux
	here="$(pwd)"
	scratch="$here/scratch"
	echo "Making scratch dir in $scratch ..."
	mkdir -p "$scratch/home"
	me="$(id -un)"

	function kill_pid () {
		pid="$1"
		desc="$2"
		if [ -n "$pid" ]; then
			echo "Killing $desc ($pid)" >&2
			# wait "$pid"
			# set -x
			kill -INT "$pid" || true
			while kill -0 "$pid" 2>/dev/null; do
				sleep 0.2
			done
		fi
	}

	dbus_pid=""
	xephyr_pid=""

	function cleanup () {
		kill_pid "$dbus_pid" DBus
		kill_pid "$xephyr_pid" Xephyr
	}

	trap cleanup EXIT

	exec 3> "$scratch/dbus-pid"
	exec 4> "$scratch/dbus-address"
	exec 5> "$scratch/xephyr-display"

	env XDG_DATA_DIRS="${
			lib.concatStringsSep ":" (map (x: "${x}/share") dbus-services)
		}''${XDG_DATA_DIRS:+:$XDG_DATA_DIRS}" \
		HOME="$here/scratch/home" \
		${dbus_daemon}/bin/dbus-daemon --session --fork \
		--print-pid=3 --print-address=4

	dbus_pid="$(cat "$scratch/dbus-pid")"
	export DBUS_SESSION_BUS_ADDRESS="$(cat "$scratch/dbus-address")"

	${xephyr} -ac -displayfd 5 -terminate "${display}" &
	xephyr_pid="$!"
	exec 5>&-
	while [ -z "$(cat scratch/xephyr-display)" ]; do
		sleep 1
		echo "awaiting xephyr..."
	done

	exec 3>&-
	exec 4>&-
	exec 5>&-

	# use sudo just to clear out envvars
	sudo -u "$me" \
		env \
		LIBGL_DRIVERS_PATH="${mesa_drivers}/lib/dri:${mesa_noglu}/lib" \
		DBUS_SESSION_BUS_ADDRESS="$(cat "$scratch/dbus-address")" \
		LD_LIBRARY_PATH="''${LD_LIBRARY_PATH:-}:${mesa_noglu}/lib" \
		HOME="$here/scratch/home" \
		DISPLAY="${display}" \
		XDG_DATA_DIRS="$scratch/home/.local/share:$HOME/.local/share:/usr/local/share:/usr/share" \
		GIO_EXTRA_MODULES="${gnome3.dconf}/lib/gio/modules" \
		PATH="${gnome3.gnome_shell}/bin:${gnome3.gnome-tweak-tool}/bin:$PATH" \
		${extraEnv} \
		"${gnome3.gnome_shell}/bin/gnome-shell" \
		"$@"
''

This works using nixpkgs xephyr now (@ NixOS/nixpkgs@696c6bed4e8)

The whole session starts very slow and some applications start very slow inside the session. This seems to be the problem: (waiting for ibus?)

** (gnome-shell:25957): WARNING **: 00:13:49.127: Error retrieving accessibility bus address: org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
GNOME Shell-Message: 00:13:49.799: Failed to launch ibus-daemon: Failed to execute child process “ibus-daemon” (No such file or directory)

I've tried to add ibus to the PATH. Then I get:

** (gnome-shell:23119): WARNING **: 00:09:44.809: Error retrieving accessibility bus address: org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
portal is not running: GDBus.Error:org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.freedesktop.portal.IBus exited with status 1

Inside the session: starting the the slow-starting applications with NO_AT_BRIDGE=1 make them fast (eg. gnome-tweaks), but adding that variable to the start-environment of the script does not work - the variable is overwritten later it seems (it's not set inside the session)

EDIT: running:

Xephyr :2  &
DISPLAY=:2 dbus-run-session gnome-shell

Works without the slowness (but might be less isolated)