feat: turn off KDE's middle-click paste

Inside a Chromium application the flag settles this on its own - with
autoscroll on, Blink stops pasting the primary selection. Everywhere else
on the desktop middle click goes on pasting, which is the thing this whole
program exists to get away from.

KWin has a switch for it, on Wayland, where the paste is a protocol the
compositor either offers or does not: EnablePrimarySelection in kwinrc.
Written with kwriteconfig, the way System Settings would, because kwinrc
has a cascade behind it. What the key said before goes in the ledger, so
disable puts it back - and a key that already said false is left alone and
not recorded, so a paste the user switched off themselves is never
switched back on.

X11 has no such switch and nothing is written there.

src/lib/kde.sh

@@ -0,0 +1,167 @@
+# shellcheck shell=bash
+#
+# KDE's middle-click paste.
+#
+# Inside a Chromium application the flag settles this on its own: with
+# autoscroll on, Blink stops pasting the primary selection on middle click,
+# because the button is doing something else now. Everywhere else on the
+# desktop middle click goes on pasting, which is the behaviour this whole
+# program exists to get away from - so the desktop's own switch for it is
+# turned off too.
+#
+# KWin has such a switch, and only for a Wayland session, where the paste is a
+# protocol the compositor either offers or does not:
+#
+#   ~/.config/kwinrc, [Wayland], EnablePrimarySelection=false
+#
+# On X11 there is nothing to switch. The primary selection is part of X itself
+# and every toolkit reaches for it on its own; no one place can say no.
+#
+# KWin reads the key once, while it starts, so what is written here is true
+# from the next login on rather than straight away.
+
+MCA_KWINRC="${MCA_XDG_CONFIG}/kwinrc"
+MCA_KWIN_GROUP="Wayland"
+MCA_KWIN_KEY="EnablePrimarySelection"
+
+# The kconfig command line tools, whichever generation this session has. They
+# are what System Settings writes kwinrc with, and worth the fork: the file has
+# a cascade behind it - /etc/xdg, the kdedefaults directory - and entries a
+# distribution can mark immutable, and an edit that reaches past all that with
+# sed writes something KDE goes on to ignore.
+MCA_KDE_TOOL=''
+MCA_KDE_TOOL_LOOKED=0
+
+_mca_kde_tool() {
+	local gen
+
+	if (( ! MCA_KDE_TOOL_LOOKED )); then
+		MCA_KDE_TOOL_LOOKED=1
+		for gen in 6 5; do
+			if mca_have "kreadconfig$gen" && mca_have "kwriteconfig$gen"; then
+				MCA_KDE_TOOL="$gen"
+				break
+			fi
+		done
+	fi
+
+	[[ -n $MCA_KDE_TOOL ]] || return 1
+	printf '%s\n' "$MCA_KDE_TOOL"
+}
+
+# mca_kde_session
+# Whether this is a KWin Wayland session.
+mca_kde_session() {
+	local desktop="${XDG_CURRENT_DESKTOP:-}"
+
+	# The watcher is a systemd service, and one started before the session
+	# imported its environment sees none of these variables. A running
+	# kwin_wayland answers both halves of the question at once, and is what
+	# such a run ends up going by.
+	if [[ -z $desktop && -z ${KDE_FULL_SESSION:-} ]]; then
+		mca_have pgrep || return 1
+		pgrep -u "$(id -u)" -x kwin_wayland > /dev/null 2>&1
+		return
+	fi
+
+	# XDG_CURRENT_DESKTOP is the one every desktop sets, and the one to go by
+	# wherever it is set; KDE_FULL_SESSION is what answers when it is not.
+	[[ -z $desktop || ${desktop^^} == *KDE* ]] || return 1
+
+	[[ ${XDG_SESSION_TYPE:-} == wayland || -n ${WAYLAND_DISPLAY:-} ]]
+}
+
+mca_kde_available() {
+	mca_kde_session && _mca_kde_tool > /dev/null
+}
+
+# mca_kde_read [file]
+# What the key says now, lowercased, empty when it is not set at all.
+mca_kde_read() {
+	local file="${1:-$MCA_KWINRC}" gen val
+	gen="$(_mca_kde_tool)" || return 1
+
+	# kreadconfig creates the file it is pointed at, and the status screen -
+	# which asks on every redraw - has no business writing anything. A file
+	# that is not there has no value in it to read anyway.
+	[[ -f $file ]] || return 0
+
+	val="$("kreadconfig$gen" --file "$file" --group "$MCA_KWIN_GROUP" \
+		--key "$MCA_KWIN_KEY" 2>/dev/null)"
+	printf '%s\n' "${val,,}"
+}
+
+# mca_kde_write [file] <value>
+# An empty value takes the key back out rather than writing nothing into it -
+# a key that is missing and a key that says nothing are not the same thing to
+# KDE, and only the first one is what an untouched kwinrc looks like.
+mca_kde_write() {
+	local file="$1" value="$2" gen
+	gen="$(_mca_kde_tool)" || return 1
+
+	if [[ -z $value ]]; then
+		"kwriteconfig$gen" --file "$file" --group "$MCA_KWIN_GROUP" \
+			--key "$MCA_KWIN_KEY" --delete 2>/dev/null
+	else
+		"kwriteconfig$gen" --file "$file" --group "$MCA_KWIN_GROUP" \
+			--key "$MCA_KWIN_KEY" "$value" 2>/dev/null
+	fi
+}
+
+# mca_kde_paste_apply
+# Turns the paste off, and remembers what the key said before so that undoing
+# puts that back rather than a guess.
+#
+# A key that already says false without the ledger knowing about it was set by
+# the user or by the distribution. That one is left alone and nothing is
+# recorded, so `disable` cannot switch a paste back on that this program never
+# switched off.
+mca_kde_paste_apply() {
+	local current previous changed=0
+
+	mca_kde_available || return 1
+	current="$(mca_kde_read)" || return 1
+
+	if mca_ledger_has "$MCA_KWINRC"; then
+		# Ours already. What goes back is the value recorded the first time
+		# round, not what reading the file now would say - that is false, and
+		# false is what is being undone.
+		previous="$(mca_ledger_detail "$MCA_KWINRC")" || previous=unset
+	elif [[ $current == false ]]; then
+		return 0
+	else
+		previous="${current:-unset}"
+	fi
+
+	if [[ $current != false ]]; then
+		mca_kde_write "$MCA_KWINRC" false || return 1
+		MCA_CHANGES=$(( MCA_CHANGES + 1 ))
+		changed=1
+	fi
+
+	mca_ledger_add kwin "$MCA_KWINRC" "$previous"
+
+	(( changed )) && mca_note "$(mca_msg "Middle click stops pasting at the next login.")"
+	return 0
+}
+
+# mca_kde_paste_revert <file> <previous value>
+mca_kde_paste_revert() {
+	local file="$1" previous="$2"
+
+	_mca_kde_tool > /dev/null || return 1
+	[[ -f $file ]] || return 1
+
+	# Only what is still recognisably ours. A key somebody has since put a
+	# value of their own into is theirs now.
+	[[ "$(mca_kde_read "$file")" == false ]] || return 1
+
+	[[ $previous == unset ]] && previous=''
+	mca_kde_write "$file" "$previous" || return 1
+
+	# Nothing was in the file but the key, which means there was no file before
+	# this program wrote one. An empty kwinrc says nothing that its absence
+	# does not say better.
+	[[ -s $file ]] || rm -f -- "$file"
+	return 0
+}