fix: recognise a wrapper script in front of Steam as Steam

A script somebody puts in front of the client to add a switch of its own is
still a way of starting Steam, but nothing recognised it as one: the check is
by name, and the script is not called steam.

Such a script tends to name the flag it exists to add, and that flag is one of
the markers the hint scan reads as "this is Chromium". The entry was therefore
handled as an application and had the flag appended to every Exec line of it,
where Steam ignores it, instead of being handed to the Steam module - so the
interface it was pointing at never got autoscroll, and the launcher entry never
got Steam's own switch either.

The handover a launcher script makes is now followed one step, and a script
that hands over to Steam counts as Steam. One step is all there is: the
client's own launcher is recognised by name already.

src/lib/detect.sh

@@ -223,6 +223,46 @@ mca_exec_is_steam_link() {
 	mca_prog_is_steam "$prog"
 }
 
+# _mca_prog_is_steam_name <program>
+# The launcher under one of the names Valve and the distributions give it.
+_mca_prog_is_steam_name() {
+	case "${1##*/}" in
+		steam|steam-runtime|steam-native|steam-jupiter) return 0 ;;
+	esac
+	return 1
+}
+
+# _mca_prog_is_steam_wrapper <program>
+# Whether a script in front of the client is a way of starting Steam. People
+# put one there to add a switch of their own, and an entry pointing at it is a
+# Steam start like any other - but the script is not named after Steam, so
+# nothing above recognises it.
+#
+# Getting this wrong is worse than it sounds: such a script tends to mention
+# the flag it is there to add, which is one of the markers that say "Chromium"
+# to the hint scan. The entry then ends up being handled as an application and
+# is given the flag on its command line, where Steam ignores it, instead of
+# being handed to the Steam module that knows how to reach the web helper.
+#
+# Only the handover is followed, and only one step of it: the client's own
+# launcher is already recognised by name, so a wrapper in front of it is the
+# whole of what is left.
+_mca_prog_is_steam_wrapper() {
+	local prog="$1" head='' target
+
+	[[ $prog == /* && -f $prog && -r $prog ]] || return 1
+
+	# Two characters, read in the shell: the scan asks this about every program
+	# on the system, and most of them are binaries whose first line is the whole
+	# file.
+	read -r -N 2 head < "$prog" 2>/dev/null || return 1
+	[[ $head == '#!' ]] || return 1
+
+	target="$(_mca_script_target "$prog")" || return 1
+	[[ -n $target ]] || return 1
+	_mca_prog_is_steam_name "$target"
+}
+
 # mca_prog_is_steam <program>
 # Whether running this program starts the Steam client. Every packaging is in
 # here and every name Valve and the distributions give the launcher, because
@@ -234,10 +274,9 @@ mca_exec_is_steam_link() {
 mca_prog_is_steam() {
 	local prog="$1"
 
-	case "${prog##*/}" in
-		steam|steam-runtime|steam-native|steam-jupiter) return 0 ;;
-	esac
-	[[ $prog == flatpak:com.valvesoftware.Steam || $prog == snap:steam ]]
+	_mca_prog_is_steam_name "$prog" && return 0
+	[[ $prog == flatpak:com.valvesoftware.Steam || $prog == snap:steam ]] && return 0
+	_mca_prog_is_steam_wrapper "$prog"
 }
 
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