diff --git a/app/src/main/java/com/droiddeck/launcher/session/SessionFiles.kt b/app/src/main/java/com/droiddeck/launcher/session/SessionFiles.kt
index f69e4c837..9bc9fc195 100644
--- a/app/src/main/java/com/droiddeck/launcher/session/SessionFiles.kt
+++ b/app/src/main/java/com/droiddeck/launcher/session/SessionFiles.kt
@@ -72,6 +72,7 @@ object SessionFiles {
             "usr/local/bin/droiddeck-steam-desktop-ui" to "usr/local/bin/droiddeck-steam-desktop-ui",
             "usr/local/bin/droiddeck-steam-library" to "usr/local/bin/droiddeck-steam-library",
             "usr/local/bin/droiddeck-seed-redists" to "usr/local/bin/droiddeck-seed-redists",
+            "usr/local/bin/droiddeck-launcher-cores" to "usr/local/bin/droiddeck-launcher-cores",
             "usr/local/bin/droiddeck-proton-extra" to "usr/local/bin/droiddeck-proton-extra",
             "usr/local/bin/droiddeck-netmanager" to "usr/local/bin/droiddeck-netmanager",
             "usr/local/bin/droiddeck-login1" to "usr/local/bin/droiddeck-login1",
diff --git a/tools/linuxfs/overlay/usr/local/bin/droiddeck-game-env b/tools/linuxfs/overlay/usr/local/bin/droiddeck-game-env
index 067212e34..792a7676a 100755
--- a/tools/linuxfs/overlay/usr/local/bin/droiddeck-game-env
+++ b/tools/linuxfs/overlay/usr/local/bin/droiddeck-game-env
@@ -255,8 +255,9 @@ EVALUATOR_SCRIPT = re.compile(r"(?:^|[\\/])evaluatorscript_[0-9]+\.vdf$", re.IGN
 
 def mark_install_scripts(command, env):
     """The client is about to run a game's install scripts (iscriptevaluator.exe): mark every process
-    in them as already run first, so no redistributable installer starts - on first launch the
-    shared ones may still be downloading, and on ARM64 Proton the VC++ ones stop with an error."""
+    in them but a Vulkan runtime installer as already run first, so no redistributable installer
+    starts - on first launch the shared ones may still be downloading, and on ARM64 Proton the VC++
+    ones stop with an error."""
     if not any(arg.lower().replace("\\", "/").endswith("iscriptevaluator.exe") for arg in command):
         return
     root = env.get("STEAM_COMPAT_CLIENT_INSTALL_PATH") or str(Path.home() / ".local/share/Steam")
diff --git a/tools/linuxfs/overlay/usr/local/bin/droiddeck-launcher-cores b/tools/linuxfs/overlay/usr/local/bin/droiddeck-launcher-cores
new file mode 100755
index 000000000..dad3d1033
--- /dev/null
+++ b/tools/linuxfs/overlay/usr/local/bin/droiddeck-launcher-cores
@@ -0,0 +1,230 @@
+#!/usr/bin/python3
+"""Holds a game launcher's hidden web interface on one slow core while something else has focus.
+
+Rockstar's launcher keeps drawing its Chromium interface (SocialClubHelper.exe: a renderer, and a
+GPU process compositing through DXVK) for as long as it runs, hidden behind the game too, and at a
+higher priority than the game's (Wine passes Chromium's on: the GPU process ran at nice -10). With
+Red Dead Redemption 2 in front on the RedMagic that was 1.3 to 2.5 cores under FEX, on a phone
+already held back by its temperature, for a page nobody sees. Held, riding at the throttle limit
+went from 46.5 to 48.7 fps, 1% lows from 27.5 to 32.6. So while the focused window belongs to
+anything but the launcher, every helper thread runs on one core of the slowest cluster at nice 19;
+when the launcher's own window has focus again (signing in, the game closed) each thread gets its
+own mask and priority back. Stopping the helpers instead would not hold: proot, the tracer of every
+guest process, resumes a stopped tracee.
+
+Focus comes from droiddeck-agent's focus.json (the path is the argument), which the agent rewrites
+whenever gamescope's focus changes. droiddeck-session starts this for Steam sessions;
+BL_LAUNCHER_CORES=0 leaves the launchers alone."""
+import json
+import os
+import signal
+import sys
+import time
+from pathlib import Path
+
+# Image paths (lower case, backslashes), matched at the end: the helpers held back, and the
+# programs whose window having focus means the user is looking at the launcher.
+HELPERS = ("\\rockstar games\\social club\\socialclubhelper.exe",)
+FRONTS = HELPERS + ("\\rockstar games\\launcher\\launcher.exe",)
+NICE = 19
+POLL = 2.0
+# How often a held launcher is looked over again: for a helper the unheld Launcher.exe started
+# since, and for threads the program has moved back.
+RESCAN = 10.0
+STEAM_CLIENT = 769
+
+
+def log(message):
+    print("droiddeck-launcher-cores: " + message, flush=True)
+
+
+def image(proc, pid):
+    """A process's program as Wine names it (argv[0]), lower case with backslashes, or None."""
+    try:
+        with open(proc / str(pid) / "cmdline", "rb") as stream:
+            first = stream.read(4096).split(b"\0", 1)[0]
+    except OSError:
+        return None
+    return first.decode("utf-8", "replace").lower().replace("/", "\\")
+
+
+def is_one_of(path, suffixes):
+    return path is not None and path.endswith(suffixes)
+
+
+def tasks(proc, pid):
+    try:
+        return [int(tid) for tid in os.listdir(proc / str(pid) / "task") if tid.isdigit()]
+    except OSError:
+        return []
+
+
+class Cores:
+    """The kernel calls, apart so a test can stand in for them."""
+
+    def own(self):
+        return os.sched_getaffinity(0)
+
+    def get(self, tid):
+        return os.sched_getaffinity(tid), os.getpriority(os.PRIO_PROCESS, tid)
+
+    def set(self, tid, cpus, nice):
+        try:
+            os.sched_setaffinity(tid, cpus)
+        except OSError:
+            # A mask outside the app's cpuset now (Android shrinks it in the background) is
+            # EINVAL; the session's own is always allowed.
+            os.sched_setaffinity(tid, self.own())
+        os.setpriority(os.PRIO_PROCESS, tid, nice)
+
+
+class Holder:
+    def __init__(self, proc=Path("/proc"), cpufreq=Path("/sys/devices/system/cpu"), cores=None):
+        self.proc = proc
+        self.cpufreq = cpufreq
+        self.cores = cores or Cores()
+        self.held = {}  # pid -> {tid: (cpus, nice) before the hold}
+        self.core = None
+
+    def slow_core(self):
+        """The last core of the slowest cluster the session has - not of the game's cores, which
+        the launcher inherits when BL_GAME_CPUS is set: the core the game misses least."""
+        def top(cpu):
+            try:
+                return int((self.cpufreq / f"cpu{cpu}/cpufreq/cpuinfo_max_freq").read_text())
+            except (OSError, ValueError):
+                return 0
+        cpus = self.cores.own()
+        slowest = min(top(cpu) for cpu in cpus)
+        return max(cpu for cpu in cpus if top(cpu) == slowest)
+
+    def helpers(self):
+        for name in os.listdir(self.proc):
+            if name.isdigit() and is_one_of(image(self.proc, name), HELPERS):
+                yield int(name)
+
+    def hold(self):
+        """Every helper thread onto the slow core at NICE: new ones, and held ones the program has
+        moved since (Chromium sets its threads' priorities, and Wine passes that on)."""
+        if self.core is None:
+            self.core = self.slow_core()
+        hold = ({self.core}, NICE)
+        for pid in self.helpers():
+            before = self.held.setdefault(pid, {})
+            for tid in tasks(self.proc, pid):
+                try:
+                    now = self.cores.get(tid)
+                    if now != hold:
+                        self.cores.set(tid, *hold)
+                except OSError:
+                    continue  # the thread ended
+                if tid not in before:
+                    # A thread that already has the hold inherited it from a held parent: it gets
+                    # what its process had, or the helpers' first.
+                    before[tid] = now if now != hold else before.get(pid) or self.first() or now
+        for pid in [pid for pid in self.held if not (self.proc / str(pid)).exists()]:
+            del self.held[pid]
+
+    def first(self):
+        for before in self.held.values():
+            for state in before.values():
+                if state != ({self.core}, NICE):
+                    return state
+        return None
+
+    def release(self):
+        """Every held thread back to its own mask and priority; one started during the hold gets
+        what its process had."""
+        for pid, before in self.held.items():
+            default = before.get(pid) or self.first()
+            for tid in tasks(self.proc, pid):
+                state = before.get(tid, default)
+                if state is None:
+                    continue
+                try:
+                    self.cores.set(tid, *state)
+                except OSError:
+                    pass
+        self.held.clear()
+
+
+def focused(snapshot):
+    """(app id, pid) of the window gamescope has focused, from droiddeck-agent's snapshot."""
+    window = snapshot.get("focusedWindow")
+    for entry in snapshot.get("focusableWindows") or []:
+        if entry.get("window") == window:
+            return snapshot.get("focusedApp"), entry.get("pid")
+    return snapshot.get("focusedApp"), None
+
+
+class Governor:
+    def __init__(self, focus_path, holder=None, clock=time.monotonic):
+        self.focus_path = Path(focus_path)
+        self.holder = holder or Holder()
+        self.clock = clock
+        self.seen = None
+        self.app = self.pid = None
+        self.scanned = None
+        self.reported = None
+
+    def read_focus(self):
+        try:
+            stamp = self.focus_path.stat().st_mtime_ns
+        except OSError:
+            return False
+        if stamp == self.seen:
+            return False
+        try:
+            self.app, self.pid = focused(json.loads(self.focus_path.read_text()))
+        except (OSError, ValueError, AttributeError):
+            return False
+        self.seen = stamp
+        return True
+
+    def tick(self):
+        changed = self.read_focus()
+        if self.pid is None:
+            return
+        launcher_in_front = is_one_of(image(self.holder.proc, self.pid), FRONTS)
+        if launcher_in_front:
+            if self.holder.held:
+                self.holder.release()
+                self.report("released: the launcher has focus")
+            return
+        # Nothing to look for until a game has had focus: launchers only run for one, and a launcher
+        # left running after its game stays held through the Steam client's own focus.
+        if self.app in (None, 0, STEAM_CLIENT) and not self.holder.held:
+            return
+        now = self.clock()
+        if changed or self.scanned is None or now - self.scanned >= RESCAN:
+            self.scanned = now
+            self.holder.hold()
+            if self.holder.held:
+                self.report(f"{len(self.holder.held)} SocialClubHelper process(es) on core {self.holder.core} "
+                            f"at nice {NICE} while the launcher is hidden")
+
+    def report(self, message):
+        if message != self.reported:
+            self.reported = message
+            log(message)
+
+    def stop(self, *_):
+        self.holder.release()
+        sys.exit(0)
+
+
+def main(argv):
+    if len(argv) != 1:
+        print("usage: droiddeck-launcher-cores <agent focus.json>", file=sys.stderr)
+        return 2
+    governor = Governor(argv[0])
+    signal.signal(signal.SIGTERM, governor.stop)
+    signal.signal(signal.SIGHUP, governor.stop)
+    signal.signal(signal.SIGINT, governor.stop)
+    while True:
+        governor.tick()
+        time.sleep(POLL)
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv[1:]))
diff --git a/tools/linuxfs/overlay/usr/local/bin/droiddeck-seed-redists b/tools/linuxfs/overlay/usr/local/bin/droiddeck-seed-redists
index 7b430422a..053df0e93 100755
--- a/tools/linuxfs/overlay/usr/local/bin/droiddeck-seed-redists
+++ b/tools/linuxfs/overlay/usr/local/bin/droiddeck-seed-redists
@@ -17,6 +17,11 @@
 # 2015-2022 runtime is recorded as installed the way Microsoft's installers look for it: on ARM64
 # Proton the x64 bundle otherwise stops with "Microsoft Visual C++ 2015-2022 Redistributable (arm64)
 # is required", while Proton's own msvcp140/vcruntime140 already serve the game.
+#
+# A Vulkan runtime installer is the one left to run. Proton has no stand-in for it: for a game whose
+# Steam compat config says nativevulkanloader (Red Dead Redemption 2) it loads vulkan-1.dll native
+# only, and Khronos' loader is in the prefix only once that installer has run - without it the game
+# exits with STATUS_DLL_NOT_FOUND before it opens a window. The installer finishes in a second.
 set -u
 steam_root=${1:?steam root}
 LIBRARIES=${DROIDDECK_LIBRARIES-/mnt/droiddeck /mnt/droiddeck-sd}
@@ -25,14 +30,15 @@ redist="$steam_root/steamapps/common/Steamworks Shared/_CommonRedist"
 
 # Every process an install script would run, as the registry key and value the client writes once
 # it has run it. Names sit one level inside "Run Process", the key they share a level further in.
-read_scripts() { xargs -0 -r awk '
+# "read_scripts vulkan" lists only the Vulkan runtime installers, which the plain call leaves out.
+read_scripts() { xargs -0 -r awk -v want="${1:-redists}" '
   # A block whose HasRunKey is set is one process; its name is the key the block opened under, at
   # whatever depth - an install script nests it under "Run Process", the client'"'"'s evaluator script
   # for a game wraps the same blocks once more.
   FNR == 1 { depth = 0; pending = "" }
-  /^[[:space:]]*\{/ { depth++; name[depth] = pending; key[depth] = ""; pending = ""; next }
+  /^[[:space:]]*\{/ { depth++; name[depth] = pending; key[depth] = ""; vulkan[depth] = 0; pending = ""; next }
   /^[[:space:]]*\}/ {
-    if (depth > 0 && key[depth] != "" && name[depth] != "")
+    if (depth > 0 && key[depth] != "" && name[depth] != "" && vulkan[depth] == (want == "vulkan"))
       printf "%s\t%s\n", key[depth], name[depth]
     if (depth > 0) depth--
     next
@@ -40,6 +46,12 @@ read_scripts() { xargs -0 -r awk '
   {
     n = split($0, part, "\"")
     if (n >= 5 && tolower(part[2]) == "hasrunkey") { key[depth] = part[4]; next }
+    if (n >= 5 && tolower(part[2]) ~ /^process [0-9]+$/) {
+      program = tolower(part[4])
+      sub(/.*[\\\/]/, "", program)
+      if (program ~ /vulkan/) vulkan[depth] = 1
+      next
+    }
     if (n == 3) pending = part[2]
   }
 '; }
@@ -84,13 +96,17 @@ if [ "$(head -n 1 "$cache" 2>/dev/null)" != "$sig" ]; then
     done <<<"$libraries" | sort -u
   } >"$cache.tmp" && mv -f "$cache.tmp" "$cache"
 fi
-game_pairs=$(tail -n +2 "$cache" 2>/dev/null | tr '\n' '\0' | read_scripts)
 # DROIDDECK_SEED_SCRIPTS: more scripts, one path a line - droiddeck-game-env passes the client's
 # evaluator script for the game about to run its installers, read before the client looks.
-if [ -n "${DROIDDECK_SEED_SCRIPTS:-}" ]; then
-  game_pairs=$(printf '%s\n%s\n' "$game_pairs" "$(printf '%s\n' "$DROIDDECK_SEED_SCRIPTS" | tr '\n' '\0' | read_scripts)")
-fi
+game_scripts() { { tail -n +2 "$cache" 2>/dev/null; printf '%s\n' "${DROIDDECK_SEED_SCRIPTS:-}"; } | sed '/^$/d' | tr '\n' '\0'; }
+game_pairs=$(game_scripts | read_scripts)
 [ -n "$(printf '%s' "$game_pairs" | tr -d '[:space:]')" ] && pairs=$(printf '%s\n%s\n' "$pairs" "$game_pairs" | sort -u | sed '/^$/d')
+# A Vulkan runtime installer marked as run by an earlier seeding still has its marker: those come out
+# again below, so the client runs the installer on the game's next launch.
+declare -A vulkan=()
+while IFS= read -r line; do
+  [[ $line == *$'\t'* ]] && vulkan[$line]=1
+done < <(game_scripts | read_scripts vulkan)
 
 # The VC++ 2015-2022 runtime as its installers check for it, for each architecture and both registry
 # views. The version is the newest the scripts above know, so any 2015-2022 bundle finds itself
@@ -138,6 +154,38 @@ block_for() { printf '%s\n' "$@" | awk -F'\t' -v now="$now" '
   for line; do [[ $line == vcrt$'\t'* ]] && vcrt "$now"; done
 }
 
+# Takes the given markers out of a registry file again, in both views, keeping its date.
+unseed() {
+  local reg=$1 out
+  shift
+  out=$(mktemp "${reg%/*}/.system.reg.XXXXXX") || return 1
+  if printf '%s\n' "$@" | awk -F'\t' '
+    NR == FNR {
+      path = $1
+      sub(/^HKEY_LOCAL_MACHINE\\\\/, "", path)
+      if (path == $1)
+        next
+      path = tolower(path)
+      drop["[" path "]" SUBSEP tolower($2)] = 1
+      if (substr(path, 1, 10) == "software\\\\")
+        drop["[software\\\\wow6432node\\\\" substr(path, 11) "]" SUBSEP tolower($2)] = 1
+      next
+    }
+    /^\[/ { section = tolower(substr($0, 1, index($0, "]"))) }
+    /^"/ {
+      value = substr($0, 2)
+      value = tolower(substr(value, 1, index(value, "\"=") - 1))
+      if ((section SUBSEP value) in drop)
+        next
+    }
+    { print }
+  ' - "$reg" >"$out" && touch -r "$reg" "$out" && mv -f "$out" "$reg"; then
+    return 0
+  fi
+  rm -f "$out"
+  return 1
+}
+
 # Proton builds a new prefix from its template, and the client runs the install scripts in that
 # prefix moments later, so the template carries the markers too. Its date is kept: Proton reads a
 # changed one as a new template and upgrades every prefix made from the old.
@@ -158,6 +206,16 @@ for reg in "$steam_root"/steamapps/common/Proton*/files/share/default_pfx*/syste
   if [ -f "$seeded" ]; then
     while IFS= read -r line; do [[ $line == *$'\t'* ]] && had[$line]=1; done <"$seeded"
   fi
+  stale=() kept=()
+  for line in "${!had[@]}"; do
+    if [ -n "${vulkan[$line]:-}" ]; then stale+=("$line"); else kept+=("$line"); fi
+  done
+  if [ ${#stale[@]} -gt 0 ] && unseed "$reg" "${stale[@]}"; then
+    had=()
+    for line in "${kept[@]}"; do had[$line]=1; done
+    printf '%s\n' "${kept[@]}" >"$seeded"
+    echo "== took Vulkan runtime markers out of $reg"
+  fi
   missing=()
   for line in "${want[@]}"; do [ -n "${had[$line]:-}" ] || missing+=("$line"); done
   [ ${#missing[@]} -eq 0 ] && continue
diff --git a/tools/linuxfs/overlay/usr/local/bin/droiddeck-session b/tools/linuxfs/overlay/usr/local/bin/droiddeck-session
index 5822ef0be..bb977c73f 100755
--- a/tools/linuxfs/overlay/usr/local/bin/droiddeck-session
+++ b/tools/linuxfs/overlay/usr/local/bin/droiddeck-session
@@ -50,6 +50,37 @@ prepare_locale() {
 }
 [ -z "${BL_INSIDE:-}" ] && prepare_locale
 
+# What earlier sessions left in /tmp. The guest's /tmp is a directory in the rootfs, never emptied
+# by a boot, and Android ends sessions by killing them: each left its mangoapp config, the client's
+# temporary directories and shared-memory sockets, Chromium's singleton directory (the web
+# helper's, an Electron app's), a session bus socket per run and a run's start stamp - 1,400
+# entries on a device after a few weeks. Each kind is swept while nothing that could own one runs,
+# with one find for the lot.
+clear_stale_tmp() {
+  local running names=() n
+  running=$(pgrep -af 'steamwebhelper|steamrtarm64/steam|--type=|droiddeck-appimage-run|dbus-run-session|dbus-daemon.*--session|dbus-launch|kwin|plasmashell' 2>/dev/null)
+  case $running in
+    *steamwebhelper* | *steamrtarm64/steam*) ;;
+    *) names+=(-o -name 'mangohud.????????' -o -name '.com.valvesoftware.Steam.??????'
+               -o -name 'steam_chrome_shmem_uid*' -o \( -type d -empty -name 'steam??????' \)) ;;
+  esac
+  case $running in *--type=*) ;; *) names+=(-o \( -type d -name 'scoped_dir??????' \)) ;; esac
+  case $running in *droiddeck-appimage-run*) ;; *) names+=(-o -name 'droiddeck-appimage.??????') ;; esac
+  case $running in
+    *dbus-run-session* | *dbus-daemon*--session* | *dbus-launch*) ;;
+    *) names+=(-o \( -type s -name 'dbus-??????????' \)) ;;
+  esac
+  case $running in
+    *kwin* | *plasmashell*) ;;
+    *) names+=(-o \( -type d -empty -name 'plasma-csd-generator.??????' \)) ;;
+  esac
+  [ ${#names[@]} -gt 0 ] || return 0
+  n=$(find /tmp -mindepth 1 -maxdepth 1 \( "${names[@]:1}" \) -print -exec rm -rf {} + 2>/dev/null | wc -l)
+  [ "$n" -gt 0 ] && echo "== /tmp: cleared $n entries earlier sessions left"
+  return 0
+}
+[ -z "${BL_INSIDE:-}" ] && clear_stale_tmp
+
 # Until the runtime is stable, Steam's own logs are copied beside the session log at exit so a
 # failed launch can be handed over as one folder.
 collect_steam_logs() {
@@ -271,7 +302,8 @@ if [ -z "${BL_INSIDE:-}" ]; then
   # BL_MANGOAPP=0 turns it off (one community build found Valve's mangoapp crashing on Turnip).
   if [ "$mode" = steam ] && [ "${BL_MANGOAPP:-1}" = 1 ] \
      && [ "${BL_STEAM_UI:-}" != desktop ] && command -v mangoapp >/dev/null 2>&1; then
-    MANGOHUD_CONFIGFILE=$(mktemp /tmp/mangohud.XXXXXXXX)
+    # One fixed file, rewritten each session: a mktemp name per session was never removed.
+    MANGOHUD_CONFIGFILE=/tmp/droiddeck-mangohud.conf
     echo no_display > "$MANGOHUD_CONFIGFILE"
     export MANGOHUD_CONFIGFILE STEAM_USE_MANGOAPP=1 STEAM_MANGOAPP_PRESETS_SUPPORTED=1 STEAM_MANGOAPP_HORIZONTAL_SUPPORTED=1
     args+=(--mangoapp)
@@ -436,6 +468,13 @@ case $mode in
         done
       ) &
     fi
+    # Rockstar's launcher keeps drawing its hidden web interface for as long as a game of theirs
+    # runs; droiddeck-launcher-cores holds it on one slow core while it is out of sight, from
+    # gamescope's focus as the agent publishes it. BL_LAUNCHER_CORES=0 leaves launchers alone.
+    if [ "${BL_LAUNCHER_CORES:-1}" != 0 ] && [ -n "${BL_LAUNCH_DIR:-}" ] && [ -z "${BL_DESKTOP:-}" ] \
+       && [ "${BL_STEAM_UI:-}" != desktop ] && [ -x /usr/local/bin/droiddeck-launcher-cores ]; then
+      /usr/local/bin/droiddeck-launcher-cores "$BL_LAUNCH_DIR/agent/focus.json" 2>&1 &
+    fi
 
     step "registering the app's game library with Steam"
     droiddeck-steam-library "$steam_root" || true
diff --git a/tools/linuxfs/overlay/usr/local/bin/droiddeck-steam-ui-scale b/tools/linuxfs/overlay/usr/local/bin/droiddeck-steam-ui-scale
index e7f41383f..b3e009e60 100755
--- a/tools/linuxfs/overlay/usr/local/bin/droiddeck-steam-ui-scale
+++ b/tools/linuxfs/overlay/usr/local/bin/droiddeck-steam-ui-scale
@@ -1,10 +1,16 @@
 #!/usr/bin/python3
-"""Initialize Steam's own Gamescope display settings once; subsequent choices belong to Steam.
+"""Initialize Steam's own display settings once; subsequent choices belong to Steam.
 
 The UI scale, and native resolution: the client treats our gamescope as an external display and,
 unless "Enable Native Resolution on External Displays" is on, draws itself at 1920x1080 on any
 larger output - a 2340x1080 session came up as 16:9 with bars. The session's size is already the
-user's Resolution choice, so the client should use all of it."""
+user's Resolution choice, so the client should use all of it.
+
+And the screensaver over a running game. The client's defaults (steamui.so's settings descriptor)
+show screensavers while a game runs, after 300 s on AC power with no button pressed - and a stick or
+trigger held is no press. Red Dead Redemption 2's cutscenes run longer than that: the slideshow
+covered the game, and the button that woke it reached the game as well (B asked to quit to the
+desktop). Customization > Screensaver > "Show screensavers when playing a game" still turns it on."""
 import math
 import os
 from pathlib import Path
@@ -15,6 +21,8 @@ import tempfile
 
 DISPLAY = "External: gamescope|||Windowed"
 NATIVE_RESOLUTION = "EnableSteamNativeExternalResolution"
+# screensaver_while_ingame: kept in config.vdf like the Gamescope settings, under this path.
+SCREENSAVER_IN_GAME = ("Customization", "Screensaver", "WhileInGame")
 TOKEN = re.compile(r'\s+|//[^\n]*|"(?:\\.|[^"\\])*"|[{}]')
 
 
@@ -108,25 +116,43 @@ def seed_text(text, scale):
     return text[:at] + insertion + text[at:], True
 
 
-def seed_native_resolution(text):
+def seed_value(text, path, value):
+    """InstallConfigStore/<path> = value, unless the client already has a value there (the
+    user's), adding only the blocks that are missing."""
     root = parse(text).get('InstallConfigStore')
     if not isinstance(root, Block):
         raise ValueError("missing InstallConfigStore")
-    gamescope = root.get('Gamescope')
-    if gamescope is not None and not isinstance(gamescope, Block):
-        raise ValueError("invalid Gamescope settings")
-    if gamescope is not None and gamescope.get(NATIVE_RESOLUTION) is not None:
+    *sections, name = path
+    block, depth = root, 1
+    while sections:
+        child = block.get(sections[0])
+        if child is None:
+            break
+        if not isinstance(child, Block):
+            raise ValueError(f"invalid {sections[0]} settings")
+        block, depth = child, depth + 1
+        sections.pop(0)
+    if not sections and block.get(name) is not None:
         return text, False
-    entry = f'"{NATIVE_RESOLUTION}" "1"'
-    if gamescope is not None:
-        at, indent = gamescope.end, 2
-    else:
-        entry = '"Gamescope"\n{\n' + entry + '\n}'
-        at, indent = root.end, 1
-    insertion = '\n' + '\n'.join('\t' * indent + line for line in entry.splitlines()) + '\n'
+    lines = [(len(sections), f'"{name}"\t\t"{value}"')]
+    for level, section in reversed(list(enumerate(sections))):
+        lines = [(level, f'"{section}"'), (level, '{')] + lines + [(level, '}')]
+    insertion = ''.join('\t' * (depth + level) + line + '\n' for level, line in lines)
+    # Before the line the block closes on, when it has one of its own; else beside the brace.
+    at = text.rfind('\n', 0, block.end) + 1
+    if text[at:block.end].strip():
+        at, insertion = block.end, '\n' + insertion
     return text[:at] + insertion + text[at:], True
 
 
+def seed_native_resolution(text):
+    return seed_value(text, ('Gamescope', NATIVE_RESOLUTION), '1')
+
+
+def seed_no_screensaver_in_game(text):
+    return seed_value(text, SCREENSAVER_IN_GAME, '0')
+
+
 def atomic_write(path, text):
     path.parent.mkdir(parents=True, exist_ok=True)
     fd, temporary = tempfile.mkstemp(prefix='.' + path.name, dir=path.parent)
@@ -140,31 +166,36 @@ def atomic_write(path, text):
 
 
 def initialize(steam_root, width, height):
+    scale = initial_scale(width, height)
+    return seed_once(steam_root, 'droiddeck-ui-scale-initialized', lambda text: seed_text(text, scale),
+                     f'== steam: initial handheld UI scale {scale:.3f}')
+
+
+def seed_once(steam_root, marker_name, seed, message):
+    """Applies seed to config.vdf the first time only: the marker keeps a later choice - including
+    a return to the client's own default, which the client may store by dropping the key - the
+    user's."""
     config = Path(steam_root) / 'config/config.vdf'
-    marker = config.with_name('droiddeck-ui-scale-initialized')
+    marker = config.with_name(marker_name)
     if marker.exists():
         return False
     text = config.read_text() if config.exists() else '"InstallConfigStore"\n{\n}\n'
-    updated, changed = seed_text(text, initial_scale(width, height))
+    updated, changed = seed(text)
     if changed:
         atomic_write(config, updated)
-        print(f'== steam: initial handheld UI scale {initial_scale(width, height):.3f}')
+        print(message)
     atomic_write(marker, '1\n')
     return changed
 
 
 def initialize_native_resolution(steam_root):
-    config = Path(steam_root) / 'config/config.vdf'
-    marker = config.with_name('droiddeck-native-resolution-initialized')
-    if marker.exists():
-        return False
-    text = config.read_text() if config.exists() else '"InstallConfigStore"\n{\n}\n'
-    updated, changed = seed_native_resolution(text)
-    if changed:
-        atomic_write(config, updated)
-        print('== steam: native resolution on the session display')
-    atomic_write(marker, '1\n')
-    return changed
+    return seed_once(steam_root, 'droiddeck-native-resolution-initialized', seed_native_resolution,
+                     '== steam: native resolution on the session display')
+
+
+def initialize_screensaver(steam_root):
+    return seed_once(steam_root, 'droiddeck-screensaver-initialized', seed_no_screensaver_in_game,
+                     '== steam: no screensaver over a running game')
 
 
 if __name__ == '__main__':
@@ -174,9 +205,10 @@ if __name__ == '__main__':
     except (OSError, ValueError, RecursionError) as error:
         print('== steam: left UI scale unchanged: ' + str(error), file=sys.stderr)
         status = 1
-    try:
-        initialize_native_resolution(sys.argv[1])
-    except (OSError, ValueError, RecursionError) as error:
-        print('== steam: left native resolution unchanged: ' + str(error), file=sys.stderr)
-        status = 1
+    for what, step in (('native resolution', initialize_native_resolution), ('screensaver', initialize_screensaver)):
+        try:
+            step(sys.argv[1])
+        except (OSError, ValueError, RecursionError) as error:
+            print(f'== steam: left {what} unchanged: {error}', file=sys.stderr)
+            status = 1
     sys.exit(status)
diff --git a/tools/tests/test_launcher_cores.py b/tools/tests/test_launcher_cores.py
new file mode 100644
index 000000000..98656b745
--- /dev/null
+++ b/tools/tests/test_launcher_cores.py
@@ -0,0 +1,220 @@
+import json
+import os
+from pathlib import Path
+import runpy
+import tempfile
+import unittest
+
+
+BIN = Path(__file__).resolve().parents[1] / 'linuxfs/overlay/usr/local/bin'
+MODULE = runpy.run_path(str(BIN / 'droiddeck-launcher-cores'))
+Holder = MODULE['Holder']
+Governor = MODULE['Governor']
+NICE = MODULE['NICE']
+
+HELPER = 'C:\\Program Files\\Rockstar Games\\Social Club\\SocialClubHelper.exe'
+LAUNCHER = 'C:\\Program Files\\Rockstar Games\\Launcher\\Launcher.exe'
+GAME = 'S:\\steamapps\\common\\Red Dead Redemption 2\\RDR2.exe'
+ALL = set(range(8))
+
+
+class FakeCores:
+    """Per-thread masks and priorities, with every change recorded."""
+
+    def __init__(self):
+        self.state = {}
+        self.calls = []
+
+    def own(self):
+        return set(ALL)
+
+    def get(self, tid):
+        if tid not in self.state:
+            raise ProcessLookupError(tid)
+        return self.state[tid]
+
+    def set(self, tid, cpus, nice):
+        if tid not in self.state:
+            raise ProcessLookupError(tid)
+        self.calls.append((tid, set(cpus), nice))
+        self.state[tid] = (set(cpus), nice)
+
+
+class LauncherCoresTest(unittest.TestCase):
+    def setUp(self):
+        temp = tempfile.TemporaryDirectory()
+        self.addCleanup(temp.cleanup)
+        self.root = Path(temp.name)
+        self.proc = self.root / 'proc'
+        self.proc.mkdir()
+        cpufreq = self.root / 'cpu'
+        for cpu in ALL:  # 0-5 one cluster, 6-7 the fast one
+            (cpufreq / f'cpu{cpu}/cpufreq').mkdir(parents=True)
+            (cpufreq / f'cpu{cpu}/cpufreq/cpuinfo_max_freq').write_text('3532800\n' if cpu < 6 else '4320000\n')
+        self.cores = FakeCores()
+        self.holder = Holder(self.proc, cpufreq, self.cores)
+        self.focus = self.root / 'focus.json'
+        self.now = 0.0
+        self.governor = Governor(self.focus, self.holder, clock=lambda: self.now)
+
+    def process(self, pid, argv0, threads=(), cpus=ALL, nice=0):
+        directory = self.proc / str(pid)
+        (directory / 'task').mkdir(parents=True)
+        (directory / 'cmdline').write_bytes(argv0.encode() + b'\0--type=renderer\0')
+        for tid in (pid, *threads):
+            self.thread(pid, tid, cpus, nice)
+
+    def thread(self, pid, tid, cpus=ALL, nice=0):
+        (self.proc / str(pid) / 'task' / str(tid)).mkdir()
+        self.cores.state[tid] = (set(cpus), nice)
+
+    def end(self, pid):
+        for tid in os.listdir(self.proc / str(pid) / 'task'):
+            self.cores.state.pop(int(tid), None)
+            (self.proc / str(pid) / 'task' / tid).rmdir()
+        (self.proc / str(pid) / 'task').rmdir()
+        (self.proc / str(pid) / 'cmdline').unlink()
+        (self.proc / str(pid)).rmdir()
+
+    def focus_on(self, pid, app=1174180):
+        stamp = getattr(self, 'stamp', 1_000_000_000) + 1_000_000_000
+        self.stamp = stamp
+        self.focus.write_text(json.dumps({
+            'focusedApp': app, 'focusedWindow': 0x200001,
+            'focusableWindows': [{'window': 0x100001, 'appId': 769, 'pid': 9},
+                                 {'window': 0x200001, 'appId': app, 'pid': pid}]}))
+        os.utime(self.focus, ns=(stamp, stamp))
+
+    def held(self, tid):
+        return self.cores.state[tid] == ({5}, NICE)
+
+    def test_helpers_held_on_the_slow_clusters_last_core_while_the_game_has_focus(self):
+        self.process(100, LAUNCHER, (101,))
+        self.process(200, HELPER, (201, 202), nice=-2)
+        self.process(300, HELPER, (301,))
+        self.process(400, GAME, (401,))
+        self.focus_on(400)
+        self.governor.tick()
+        for tid in (200, 201, 202, 300, 301):
+            self.assertTrue(self.held(tid), tid)
+        for tid in (100, 101, 400, 401):
+            self.assertEqual((ALL, 0), self.cores.state[tid])
+
+    def test_launcher_focus_gives_each_thread_its_own_mask_and_priority_back(self):
+        self.process(100, LAUNCHER)
+        self.process(200, HELPER, (201,), cpus={0, 1, 2, 3}, nice=-2)
+        self.cores.state[201] = ({6, 7}, 4)
+        self.process(400, GAME)
+        self.focus_on(400)
+        self.governor.tick()
+        self.assertTrue(self.held(201))
+        self.focus_on(100)
+        self.governor.tick()
+        self.assertEqual(({0, 1, 2, 3}, -2), self.cores.state[200])
+        self.assertEqual(({6, 7}, 4), self.cores.state[201])
+        self.assertEqual({}, self.holder.held)
+
+    def test_nothing_is_touched_before_a_game_has_focus(self):
+        self.process(100, LAUNCHER)
+        self.process(200, HELPER)
+        self.process(9, '/root/.local/share/Steam/steamrtarm64/steam')
+        self.governor.tick()  # no focus file yet
+        self.focus_on(9, app=769)
+        self.governor.tick()
+        self.focus_on(100)
+        self.governor.tick()
+        self.assertEqual([], self.cores.calls)
+
+    def test_a_launcher_left_after_its_game_stays_held_behind_the_client(self):
+        self.process(200, HELPER)
+        self.process(400, GAME)
+        self.process(9, '/root/.local/share/Steam/steamrtarm64/steam')
+        self.focus_on(400)
+        self.governor.tick()
+        self.end(400)
+        self.focus_on(9, app=769)
+        self.governor.tick()
+        self.assertTrue(self.held(200))
+
+    def test_a_thread_inheriting_the_hold_is_released_to_what_its_process_had(self):
+        self.process(200, HELPER, cpus={0, 1, 2, 3, 4, 5}, nice=1)
+        self.process(400, GAME)
+        self.focus_on(400)
+        self.governor.tick()
+        # A new thread, and a new helper process the held one started: both born held.
+        self.thread(200, 203, {5}, NICE)
+        self.process(500, HELPER, (501,), cpus={5}, nice=NICE)
+        self.now += 10
+        self.governor.tick()
+        self.focus_on(200)
+        self.governor.tick()
+        for tid in (200, 203, 500, 501):
+            self.assertEqual(({0, 1, 2, 3, 4, 5}, 1), self.cores.state[tid], tid)
+
+    def test_a_held_thread_the_program_moves_back_is_held_again_on_the_next_look(self):
+        self.process(200, HELPER, (201,))
+        self.process(400, GAME)
+        self.focus_on(400)
+        self.governor.tick()
+        self.cores.state[201] = (ALL, -5)  # the program raised a thread's priority itself
+        self.now += 5
+        self.governor.tick()
+        self.assertFalse(self.held(201))  # not looked at again before the rescan
+        self.now += 5
+        self.governor.tick()
+        self.assertTrue(self.held(201))
+        self.focus_on(200)
+        self.governor.tick()
+        self.assertEqual((ALL, 0), self.cores.state[201])  # its first state, not the program's
+
+    def test_a_steady_hold_is_not_rewritten(self):
+        self.process(200, HELPER, (201,))
+        self.process(400, GAME)
+        self.focus_on(400)
+        self.governor.tick()
+        calls = len(self.cores.calls)
+        for _ in range(5):
+            self.now += 10
+            self.governor.tick()
+        self.assertEqual(calls, len(self.cores.calls))
+
+    def test_threads_that_end_are_skipped_and_ended_helpers_forgotten(self):
+        self.process(200, HELPER, (201,))
+        self.process(300, HELPER)
+        self.process(400, GAME)
+        del self.cores.state[201]  # ended between the listing and the call
+        self.focus_on(400)
+        self.governor.tick()
+        self.assertNotIn(201, self.holder.held[200])
+        self.end(300)
+        self.now += 10
+        self.governor.tick()
+        self.assertEqual([200], list(self.holder.held))
+
+    def test_stop_releases(self):
+        self.process(200, HELPER)
+        self.process(400, GAME)
+        self.focus_on(400)
+        self.governor.tick()
+        with self.assertRaises(SystemExit):
+            self.governor.stop()
+        self.assertEqual((ALL, 0), self.cores.state[200])
+
+    def test_unreadable_focus_is_ignored(self):
+        self.process(200, HELPER)
+        self.focus.write_text('{"focusedApp": 1174')
+        self.governor.tick()
+        self.focus.write_text('[]')
+        os.utime(self.focus, ns=(5, 5))
+        self.governor.tick()
+        self.assertEqual([], self.cores.calls)
+
+    def test_window_without_a_pid_is_left_alone(self):
+        self.process(200, HELPER)
+        self.focus.write_text(json.dumps({'focusedApp': 1174180, 'focusedWindow': 7, 'focusableWindows': []}))
+        self.governor.tick()
+        self.assertEqual([], self.cores.calls)
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/tools/tests/test_seed_redists.py b/tools/tests/test_seed_redists.py
index abc77ab07..1f9e2ae71 100644
--- a/tools/tests/test_seed_redists.py
+++ b/tools/tests/test_seed_redists.py
@@ -22,6 +22,30 @@ GAME_SCRIPT = r'''"InstallScript"
 }
 '''
 
+# Red Dead Redemption 2's own script: Proton needs Khronos' vulkan-1.dll for it, which only the
+# Vulkan runtime installer puts in the prefix.
+VULKAN_SCRIPT = r'''"InstallScript"
+{
+	"Run Process"
+	{
+		"Vulkan111080"
+		{
+			"HasRunKey"		"HKEY_LOCAL_MACHINE\\Software\\Valve\\Steam\\Apps\\1174180"
+			"process 1"		"%INSTALLDIR%\\Redistributables\\VulkanRT-1.1.108.0-Installer.exe"
+			"command 1"		"/S"
+			"NoCleanUp"		"0"
+		}
+		"RockstarLauncher"
+		{
+			"HasRunKey"		"HKEY_LOCAL_MACHINE\\Software\\Rockstar Games\\Steam\\Launcher"
+			"process 1"		"%INSTALLDIR%\\Redistributables\\Rockstar-Games-Launcher.exe"
+			"command 1"		"/s /t"
+			"NoCleanUp"		"0"
+		}
+	}
+}
+'''
+
 
 class SeedRedistsTest(unittest.TestCase):
     def setUp(self):
@@ -91,6 +115,45 @@ class SeedRedistsTest(unittest.TestCase):
         self.run_seed()
         self.assertEqual(int(self.reg.stat().st_mtime), 1_000_000_000)
 
+    def rdr2(self):
+        path = self.steam / "steamapps/common/Red Dead Redemption 2/installscript.vdf"
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(VULKAN_SCRIPT)
+        (self.steam / "steamapps/appmanifest_1174180.acf").write_text('"AppState" { "installdir" "Red Dead Redemption 2" }')
+
+    def test_a_vulkan_runtime_installer_is_left_to_run(self):
+        self.rdr2()
+        self.run_seed()
+        reg = self.reg.read_text()
+        self.assertNotIn("Vulkan111080", reg)
+        self.assertIn('"RockstarLauncher"=dword:00000001', reg)
+        self.assertNotIn("Vulkan111080", (self.reg.parent / ".droiddeck-redists").read_text())
+
+    def test_a_vulkan_marker_an_earlier_seeding_wrote_comes_out_once(self):
+        self.rdr2()
+        marker = "HKEY_LOCAL_MACHINE\\\\Software\\\\Valve\\\\Steam\\\\Apps\\\\1174180\tVulkan111080"
+        self.reg.write_text(
+            "WINE REGISTRY Version 2\n"
+            "\n[Software\\\\Valve\\\\Steam\\\\Apps\\\\1174180] 1791729070\n#time=1dd598d29161f72\n"
+            '"Other"=dword:00000001\n"Vulkan111080"=dword:00000001\n'
+            "\n[Software\\\\Wow6432Node\\\\Valve\\\\Steam\\\\Apps\\\\1174180] 1791729070\n"
+            '"Vulkan111080"=dword:00000001\n'
+            "\n[Software\\\\Valve\\\\Steam\\\\Apps\\\\22380] 1791311644\n"
+            '"Vulkan111080"=dword:00000001\n')
+        (self.reg.parent / ".droiddeck-redists").write_text(marker + "\n")
+        os.utime(self.reg, (1_000_000_000, 1_000_000_000))
+        self.run_seed()
+        reg = self.reg.read_text()
+        self.assertEqual(reg.count('"Vulkan111080"'), 1, "only the other app's value stays")
+        self.assertIn('"Other"=dword:00000001', reg)
+        self.assertNotIn("Vulkan111080", (self.reg.parent / ".droiddeck-redists").read_text())
+        self.assertEqual(int(self.reg.stat().st_mtime), 1_000_000_000)
+        # The client writes the marker itself once the installer has run; that one is left alone.
+        with self.reg.open("a") as out:
+            out.write('\n[Software\\\\Valve\\\\Steam\\\\Apps\\\\1174180] 1791730000\n"Vulkan111080"=dword:00000001\n')
+        self.run_seed()
+        self.assertEqual(self.reg.read_text().count('"Vulkan111080"'), 2)
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/tools/tests/test_session_tmp.py b/tools/tests/test_session_tmp.py
new file mode 100644
index 000000000..58607911d
--- /dev/null
+++ b/tools/tests/test_session_tmp.py
@@ -0,0 +1,95 @@
+import os
+import re
+import shutil
+import socket
+import subprocess
+import tempfile
+import unittest
+from pathlib import Path
+
+SESSION = Path(__file__).resolve().parents[1] / "linuxfs/overlay/usr/local/bin/droiddeck-session"
+
+STEAM = ["mangohud.Ab3dE6gH", ".com.valvesoftware.Steam.x1Y2z3/", "steam_chrome_shmem_uid10480_0", "steamADfAnd/"]
+CHROMIUM = ["scoped_dir3yu9Xt/SingletonSocket"]
+APPIMAGE = ["droiddeck-appimage.19PoOD"]
+BUS = ["dbus-1zWfOP1IaR="]  # = a socket
+KDE = ["plasma-csd-generator.BVWIWA/"]
+STALE = STEAM + CHROMIUM + APPIMAGE + BUS + KDE
+KEPT = ["droiddeck-mangohud.conf", ".X11-unix/", "steamworks", "steamABCDEF/inside", "notes.txt",
+        "mangohud.conf", ".wine-10480/", "dbus-1zWfOP1IaR.log", "dbus-notasocket",
+        "plasma-csd-generator.CQGZSw/kept"]
+
+
+def sweep_function(tmp):
+    match = re.search(r"^clear_stale_tmp\(\) \{\n.*?^\}\n", SESSION.read_text(), re.M | re.S)
+    return match.group(0).replace("find /tmp ", "find %s " % tmp)
+
+
+class SessionTmpTest(unittest.TestCase):
+    def setUp(self):
+        self.root = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.root, True)
+        self.tmp = self.root / "tmp"
+        self.tmp.mkdir()
+        for name in STALE + KEPT:
+            path = self.tmp / name
+            if name.endswith("/"):
+                path.mkdir(parents=True)
+            elif name.endswith("="):
+                listener = socket.socket(socket.AF_UNIX)
+                listener.bind(str(self.tmp / name[:-1]))
+                listener.close()
+            else:
+                path.parent.mkdir(parents=True, exist_ok=True)
+                path.write_text("x")
+        self.bin = self.root / "bin"
+        self.bin.mkdir()
+
+    def sweep(self, running=""):
+        pgrep = self.bin / "pgrep"
+        pgrep.write_text("#!/bin/sh\nprintf '%s' '" + running + "'\n")
+        pgrep.chmod(0o755)
+        env = dict(os.environ, PATH="%s:%s" % (self.bin, os.environ["PATH"]))
+        return subprocess.run(["bash", "-c", sweep_function(self.tmp) + "clear_stale_tmp"], env=env,
+                              capture_output=True, text=True, timeout=30, check=True).stdout
+
+    def exists(self, name):
+        return os.path.lexists(self.tmp / name.rstrip("=").split("/")[0])
+
+    def test_what_killed_sessions_left_goes_and_nothing_else(self):
+        self.assertEqual("== /tmp: cleared 8 entries earlier sessions left\n", self.sweep())
+        for name in STALE:
+            self.assertFalse(self.exists(name), name)
+        for name in KEPT:
+            self.assertTrue((self.tmp / name).exists(), name)
+
+    def kept_while(self, running, owned):
+        self.sweep(running)
+        for name in STALE:
+            self.assertEqual(name in owned, self.exists(name), name)
+
+    def test_a_running_client_keeps_its_own(self):
+        self.kept_while("1234 /root/.local/share/Steam/steamrtarm64/steam -gamepadui\n", STEAM)
+
+    def test_a_running_chromium_keeps_its_singleton(self):
+        self.kept_while("88 /opt/Heroic/heroic --type=renderer --enable-features=x\n", CHROMIUM)
+
+    def test_a_running_appimage_keeps_its_stamp(self):
+        self.kept_while("77 /bin/bash /usr/local/bin/droiddeck-appimage-run /root/Apps/X.AppImage\n", APPIMAGE)
+
+    def test_a_running_session_bus_keeps_its_socket(self):
+        self.kept_while("66 dbus-daemon --config-file=/usr/share/dbus-1/session.conf --nofork --session\n", BUS)
+
+    def test_the_system_bus_is_not_a_session_bus(self):
+        self.kept_while("55 dbus-daemon --config-file=/usr/local/share/droiddeck/system-bus.conf --nofork\n", [])
+
+    def test_a_running_desktop_keeps_its_decorations(self):
+        self.kept_while("44 kwin_wayland --xwayland\n", KDE)
+
+    def test_a_clean_tmp_says_nothing(self):
+        self.sweep()
+        self.assertEqual("", self.sweep())
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/tools/tests/test_steam_ui_scale.py b/tools/tests/test_steam_ui_scale.py
index 8d4689483..5440dcd49 100644
--- a/tools/tests/test_steam_ui_scale.py
+++ b/tools/tests/test_steam_ui_scale.py
@@ -14,6 +14,8 @@ DISPLAY = MODULE['DISPLAY']
 seed_native_resolution = MODULE['seed_native_resolution']
 initialize_native_resolution = MODULE['initialize_native_resolution']
 NATIVE_RESOLUTION = MODULE['NATIVE_RESOLUTION']
+seed_no_screensaver_in_game = MODULE['seed_no_screensaver_in_game']
+initialize_screensaver = MODULE['initialize_screensaver']
 
 
 class SteamUiScaleTest(unittest.TestCase):
@@ -151,3 +153,60 @@ class SteamNativeResolutionTest(unittest.TestCase):
                 initialize_native_resolution(self.root)
             self.assertEqual(text, self.config.read_text())
             self.assertFalse(self.marker.exists())
+
+
+class SteamScreensaverTest(unittest.TestCase):
+    def setUp(self):
+        self.temp = tempfile.TemporaryDirectory()
+        self.addCleanup(self.temp.cleanup)
+        self.root = Path(self.temp.name)
+        self.config = self.root / 'config/config.vdf'
+        self.config.parent.mkdir()
+        self.marker = self.config.with_name('droiddeck-screensaver-initialized')
+
+    def in_game(self, text):
+        return parse(text).get('InstallConfigStore').get('Customization').get('Screensaver').get('WhileInGame')
+
+    def test_fresh_client_shows_no_screensaver_over_a_game(self):
+        self.assertTrue(initialize_screensaver(self.root))
+        self.assertEqual('0', self.in_game(self.config.read_text()))
+        self.assertTrue(self.marker.exists())
+
+    def test_it_goes_beside_the_screensaver_the_user_picked(self):
+        text = ('"InstallConfigStore"\n{\n\t"Customization"\n\t{\n\t\t"Screensaver"\n\t\t{\n'
+                '\t\t\t"CurrentScreensaver"\t\t"steam-bouncinglogo"\n\t\t}\n\t}\n}\n')
+        updated, changed = seed_no_screensaver_in_game(text)
+        self.assertTrue(changed)
+        screensaver = parse(updated).get('InstallConfigStore').get('Customization').get('Screensaver')
+        self.assertEqual('steam-bouncinglogo', screensaver.get('CurrentScreensaver'))
+        self.assertEqual('0', screensaver.get('WhileInGame'))
+        self.assertIn('\t\t\t"WhileInGame"\t\t"0"\n\t\t}\n', updated)
+
+    def test_users_choice_is_kept(self):
+        text = '"InstallConfigStore" { "Customization" { "Screensaver" { "WhileInGame" "1" } } }'
+        self.assertEqual((text, False), seed_no_screensaver_in_game(text))
+
+    def test_turning_it_back_on_survives_restart(self):
+        initialize_screensaver(self.root)
+        text = '"InstallConfigStore" { "Customization" { } }'
+        self.config.write_text(text)
+        self.assertFalse(initialize_screensaver(self.root))
+        self.assertEqual(text, self.config.read_text())
+
+    def test_every_seeded_setting_lands_in_one_valid_config(self):
+        initialize(self.root, 2340, 1080)
+        initialize_native_resolution(self.root)
+        initialize_screensaver(self.root)
+        store = parse(self.config.read_text()).get('InstallConfigStore')
+        self.assertIsNotNone(store.get('UI').get('display').get(DISPLAY))
+        self.assertEqual('1', store.get('Gamescope').get(NATIVE_RESOLUTION))
+        self.assertEqual('0', store.get('Customization').get('Screensaver').get('WhileInGame'))
+
+    def test_invalid_config_is_never_rewritten_or_marked_initialized(self):
+        for text in ('"InstallConfigStore" {', '"InstallConfigStore" { "Customization" "bad" }',
+                     '"InstallConfigStore" { "Customization" { "Screensaver" "bad" } }'):
+            self.config.write_text(text)
+            with self.assertRaises(ValueError):
+                initialize_screensaver(self.root)
+            self.assertEqual(text, self.config.read_text())
+            self.assertFalse(self.marker.exists())