polybar: Add calendar popup on date-module click

.config/polybar/config

@@ -120,7 +120,7 @@ interval = 1
 date = %Y-%m-%d%
 time = "%H:%M:%S  "
 
-label = "%date% - %time%"
+label = ${env:POLYBAR_DATETIME}
 
 ;#################################################
 ;#################################################

.config/polybar/launch.sh

@@ -3,11 +3,13 @@
 XDG_CONF="${XDG_CONFIG_HOME:-$HOME/.config}"
 BASE_DIR="$XDG_CONF/polybar"
 
+calendar_exec="st -A 0.45 -t polybar-datetime-calendar \"$BASE_DIR/scripts/calendar.sh\" &"
 declare -A module_flags
 module_flags=(
 	[battery]="$BASE_DIR/scripts/battery.sh"
 	[bluetooth]="$BASE_DIR/scripts/bluetooth.sh"
 	[bluetooth_click_left]="$BASE/scripts/bluetooth.sh --toggle &"
+	[datetime]="%{A1:$calendar_exec:} %date% - %time% %{A}"
 	[media]="$BASE_DIR/scripts/media.sh"
 	[powermenu]="%{A1:$XDG_CONF/rofi/powermenu.sh &:}%{T2}...%{T-}%{A}"
 	[vpn]="$BASE_DIR/scripts/vpn.sh"

.config/polybar/scripts/calendar.sh

@@ -0,0 +1,46 @@
+#!/bin/bash
+
+# The separation and recursion is sadly necessary since reload is apparently
+# executed in a subshell and thus using an environment variable does not work
+
+dn="reload(\"$0\" -)"
+up="reload(\"$0\" +)"
+FZF_ARGS=(
+	--reverse
+	--no-sort
+	--no-info
+	--phony
+	--prompt=
+	--bind="ctrl-j:$dn,down:$dn,ctrl-k:$up,up:$up,enter:abort"
+)
+
+if [[ $# -eq 0 ]]; then
+	# Frontend
+
+	# Create temporary file for the counter that should be removed when exiting.
+	export FZF_CAL_TMP="$(mktemp)"
+	trap "rm -f $FZF_CAL_TMP" EXIT
+	echo 0 > "$FZF_CAL_TMP"
+
+	# Call fzf
+	cal -wm | fzf "${FZF_ARGS[@]}"
+	exit 0
+else
+	# Backend
+
+	# + Add to counter, going back in time
+	# - Subtract from counter, going forward in time
+	# = Print current month without modifying the counter
+	[[ $1 = "+" || $1 = "-" || $1 = "=" ]] || exit 1
+	[[ -n $FZF_CAL_TMP ]] || exit 1
+
+	counter=$(cat "$FZF_CAL_TMP")
+	if [[ $1 != "=" ]]; then
+		# Increment or decrement counter
+		: $((counter$1$1))
+		echo $counter >"$FZF_CAL_TMP"
+	fi
+
+	# Print calendar
+	cal -wm $(date +"%m %Y" -d "$counter months ago")
+fi