i3: Simulate monitor-independent workspaces

Often when I want to open a new workspace, I look at my bar to see which
workspaces are still free. This does not work sometimes when having
multiple monitors connected, as workspace numbers are unique between all
outputs.

Add a script that handles workspace switching/moving by prepending the
index with the index of the current monitor.

.config/i3/multi-monitor-workspaces.sh

@@ -0,0 +1,54 @@
+#!/bin/sh
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2025 Julian Prein
+#
+# Script to simulate output-independent workspaces. Can be used to switch and/or
+# move a container to the workspace.
+
+set -eu
+
+switch=
+move=
+
+usage() {
+	printf >&2 "Usage: %s [-s] [-m] <index>\n" "$0"
+	exit "${1:-0}"
+}
+
+while getopts "sm" flag; do
+	case $flag in
+		s) switch=1;;
+		m) move=1;;
+		*) usage 1;;
+	esac
+done
+shift $((OPTIND - 1))
+[ $# -gt 0 ] || usage
+
+outputs="$(i3-msg -t get_outputs | jq -r '.[] | select(.active).name')"
+num_outs="$(printf "%s\n" "$outputs" | wc -l)"
+
+if [ "$num_outs" -lt 2 ]; then
+	# only one monitor
+	workspace="$1"
+else
+	name="$(i3-msg -t get_tree \
+		| jq -r '.. | objects | select(.focused).output')"
+	num="$(printf "%s\n" "$outputs" \
+		| grep -Fxn "$name" \
+		| cut -d: -f1)"
+	num="$((num - 1))"
+
+	# Omit the number on the first monitor
+	[ "$num" -gt 0 ] || num=
+
+	workspace="$num$1"
+fi
+
+if [ -z "$switch" ] && [ -z "$move" ]; then
+	printf "%s\n" "$workspace"
+	exit 0
+fi
+
+[ -z "$move" ] || i3-msg move container to workspace "$workspace"
+[ -z "$switch" ] || i3-msg workspace "$workspace"