From 99d9908017c98365c07f2d810d1b8a457f8e50ee Mon Sep 17 00:00:00 2001
From: AverageConsumer <35539970+AverageConsumer@users.noreply.github.com>
Date: Sun, 16 Aug 2026 15:54:04 +0200
Subject: [PATCH] render: add cross-platform desktop companion display

---
 conf.lua                               |  17 ++-
 docs/modding.md                        |   4 +
 main.lua                               |   5 +
 src/core/HostShell.lua                 |  32 ++++++
 src/render/DesktopCompanion.lua        | 125 ++++++++++++++++++++++
 src/render/DesktopScreen.lua           | 137 +++++++++++++++++++++++++
 src/render/SecondScreen.lua            |  33 ++++--
 tests/engine/desktop_companion.lua     |  56 ++++++++++
 tests/engine/desktop_second_screen.lua |  57 ++++++++++
 tests/engine/hostshell_spawn_self.lua  |  32 ++++++
 10 files changed, 490 insertions(+), 8 deletions(-)
 create mode 100644 src/render/DesktopCompanion.lua
 create mode 100644 src/render/DesktopScreen.lua
 create mode 100644 tests/engine/desktop_companion.lua
 create mode 100644 tests/engine/desktop_second_screen.lua
 create mode 100644 tests/engine/hostshell_spawn_self.lua

diff --git a/conf.lua b/conf.lua
index 2894ffe5..1fe4d5da 100644
--- a/conf.lua
+++ b/conf.lua
@@ -6,18 +6,30 @@ function love.conf(t)
 
   local editor = os.getenv("POKEPORT_EDITOR") == "1"
   local developer = os.getenv("POKEPORT_DEV") == "1"
+  local companion = nil
   if arg then
     for _, a in ipairs(arg) do
       if a == "--editor" then editor = true end
       if a == "--developer" then developer = true end
+      local port, token = a:match("^%-%-display%-companion=(%d+),([%w]+)$")
+      if port then companion = { port = tonumber(port), token = token } end
     end
   end
   -- main.lua runs in the same Lua state right after conf.lua; stash the
   -- decision in a global so it doesn't need to reparse `arg`.
   _G.POKEPORT_EDITOR_MODE = editor
   _G.POKEPORT_DEV_MODE = developer
+  _G.POKEPORT_DISPLAY_COMPANION = companion
 
-  if editor then
+  if companion then
+    t.identity = "pokemon-love2d-companion"
+    t.window.title = "gen1recomp Secondary Display"
+    t.window.width = 640
+    t.window.height = 576
+    t.window.minwidth = 160
+    t.window.minheight = 144
+    t.window.resizable = true
+  elseif editor then
     -- Same identity as the game, deliberately: the editor edits the game's
     -- saves and reads the game's ROM cache, both of which live under this
     -- folder.  A private editor identity would point love.filesystem at an
@@ -51,7 +63,8 @@ function love.conf(t)
   end
   t.version = love._os == "iOS" and "12.0" or "11.5"
   t.window.vsync = 1
-  t.modules.joystick = true
+  t.modules.audio = not companion
+  t.modules.joystick = not companion
   t.modules.physics = false
 
   -- love.system is not loaded during love.conf; love._os is set by the
diff --git a/docs/modding.md b/docs/modding.md
index 0180762d..de1726bd 100644
--- a/docs/modding.md
+++ b/docs/modding.md
@@ -738,6 +738,10 @@ palette-correct blit of either canvas into an arbitrary screen rect, and the
 oldest queued event as `"action,x,y"` in submitted-frame coordinates, or `nil`.
 This is what lets a mod lay the two passes out as two stacked Game Boy screens,
 or push one onto a second screen, without the engine knowing the layout.
+On process-capable Windows, Linux and macOS hosts without a native display
+bridge, enabling this facade opens a second resizable app window instead. It
+uses the same `available`, `detected`, `push`, `pollTouch` and `setEnabled`
+contract, so a mod does not need a desktop-specific rendering path.
 
 `render.output_enabled` and `render.output` are the later, whole-window seam
 for mods that need the engine's normal composite rather than its separate
diff --git a/main.lua b/main.lua
index c890caf9..4fbd44f2 100644
--- a/main.lua
+++ b/main.lua
@@ -8,6 +8,11 @@
 --     opens the editor on that slot's file, and restores the launcher when
 --     the editor's Close button is pressed (openEditor / closeEditor below)
 
+if POKEPORT_DISPLAY_COMPANION then
+  return require("src.render.DesktopCompanion").install(
+    POKEPORT_DISPLAY_COMPANION)
+end
+
 local editorMode = os.getenv("POKEPORT_EDITOR") == "1" or POKEPORT_EDITOR_MODE == true
 
 local SwitchDiagnostics = require("src.debug.SwitchDiagnostics")
diff --git a/src/core/HostShell.lua b/src/core/HostShell.lua
index fe19021c..9939ef39 100644
--- a/src/core/HostShell.lua
+++ b/src/core/HostShell.lua
@@ -272,6 +272,38 @@ function HostShell.quote(s)
   return "'" .. s:gsub("'", "'\\''") .. "'"
 end
 
+-- Launch another instance of this packaged app without waiting for it.  The
+-- same path works on all process-capable desktop hosts; only the shell's
+-- background spelling differs.  Source checkouts include their game folder,
+-- while fused releases and AppImages already carry it in the executable.
+function HostShell.spawnSelfDetached(args)
+  if not require("src.core.Platform").canSpawnProcess() then return false end
+  local fs = love and love.filesystem
+  if not (fs and fs.getExecutablePath) then return false end
+  local executable = os.getenv("APPIMAGE") or fs.getExecutablePath()
+  if type(executable) ~= "string" or executable == "" then return false end
+
+  local argv = {}
+  local fused = fs.isFused and fs.isFused()
+  if not os.getenv("APPIMAGE") and not fused and fs.getSource then
+    argv[#argv + 1] = fs.getSource()
+  end
+  for _, value in ipairs(args or {}) do argv[#argv + 1] = tostring(value) end
+
+  local command = HostShell.quote(executable)
+  for _, value in ipairs(argv) do
+    command = command .. " " .. HostShell.quote(value)
+  end
+  local osName = love.system and love.system.getOS and love.system.getOS()
+  if osName == "Windows" then
+    command = 'start "" /b ' .. command .. " >NUL 2>&1"
+  else
+    command = HostShell.envPrefix() .. command .. " >/dev/null 2>&1 &"
+  end
+  local ok, _, code = os.execute(command)
+  return ok == true or ok == 0 or code == 0
+end
+
 -- MEMOISED per Lua state (so once per thread).  This used to spawn a whole
 -- `curl --version` process on every single fetch -- twice for a GET through
 -- the Android-bridge fallback -- which doubled the number of spawns the lock
diff --git a/src/render/DesktopCompanion.lua b/src/render/DesktopCompanion.lua
new file mode 100644
index 00000000..0702040b
--- /dev/null
+++ b/src/render/DesktopCompanion.lua
@@ -0,0 +1,125 @@
+-- Minimal second-window process for src/render/DesktopScreen.lua.
+
+local DesktopCompanion = {}
+
+function DesktopCompanion.install(config)
+  local enet = require("enet")
+  local host = assert(enet.host_create())
+  local peer = assert(host:connect(("127.0.0.1:%d"):format(config.port), 2))
+  local image, sourceW, sourceH, preference
+  local background = { 0, 0, 0, 1 }
+  local connected, commandedQuit = false, false
+  local pointerDown = false
+  local started = love.timer.getTime()
+  local lastContact = started
+
+  local function send(kind, payload)
+    if not connected then return end
+    pcall(peer.send, peer, kind .. config.token .. (payload or ""), 1, "reliable")
+  end
+
+  local function receiveFrame(data)
+    local prefix = "F" .. config.token .. "\n"
+    if data:sub(1, #prefix) ~= prefix then return end
+    local split = data:find("\n", #prefix + 1, true)
+    if not split then return end
+    local w, h, rgb, mode = data:sub(#prefix + 1, split - 1)
+      :match("^(%d+),(%d+),(%d+),([%w_:.-]+)$")
+    w, h, rgb = tonumber(w), tonumber(h), tonumber(rgb)
+    if not w or not h or w < 1 or h < 1 or w > 4096 or h > 4096 then return end
+    local ok, raw = pcall(love.data.decompress, "string", "lz4",
+      data:sub(split + 1))
+    if not ok or type(raw) ~= "string" or #raw ~= w * h * 4 then return end
+    local made, pixels = pcall(love.image.newImageData, w, h, "rgba8", raw)
+    if not made then return end
+    if not image or sourceW ~= w or sourceH ~= h then
+      if image and image.release then image:release() end
+      image = love.graphics.newImage(pixels)
+    else
+      image:replacePixels(pixels)
+    end
+    sourceW, sourceH, preference = w, h, mode
+    image:setFilter(mode:find("cover", 1, true) and "linear" or "nearest",
+      mode:find("cover", 1, true) and "linear" or "nearest")
+    background = {
+      math.floor(rgb / 0x10000) % 0x100 / 255,
+      math.floor(rgb / 0x100) % 0x100 / 255,
+      rgb % 0x100 / 255, 1,
+    }
+  end
+
+  local function service()
+    while true do
+      local event = host:service(0)
+      if not event then break end
+      if event.type == "connect" then
+        connected, lastContact = true, love.timer.getTime()
+        send("H")
+      elseif event.type == "receive" then
+        lastContact = love.timer.getTime()
+        if event.data == "Q" .. config.token then
+          commandedQuit = true
+          love.event.quit()
+        elseif event.data ~= "P" .. config.token then
+          receiveFrame(event.data)
+        end
+      elseif event.type == "disconnect" then
+        love.event.quit()
+      end
+    end
+  end
+
+  local function placement()
+    if not image then return 0, 0, 1 end
+    local ww, wh = love.graphics.getDimensions()
+    local cover = preference and preference:find("cover", 1, true)
+    local scale = (cover and math.max or math.min)(ww / sourceW, wh / sourceH)
+    return (ww - sourceW * scale) / 2, (wh - sourceH * scale) / 2, scale
+  end
+
+  local function input(action, x, y)
+    if not image then return false end
+    local dx, dy, scale = placement()
+    local sx, sy = math.floor((x - dx) / scale), math.floor((y - dy) / scale)
+    if sx < 0 or sy < 0 or sx >= sourceW or sy >= sourceH then return false end
+    send("I", ("\n%s,%d,%d"):format(action, sx, sy))
+    return true
+  end
+
+  function love.update()
+    service()
+    local t = love.timer.getTime()
+    if (not connected and t - started > 5) or t - lastContact > 5 then
+      love.event.quit()
+    end
+  end
+
+  function love.draw()
+    love.graphics.clear(background[1], background[2], background[3], background[4])
+    if not image then return end
+    local x, y, scale = placement()
+    love.graphics.setColor(1, 1, 1, 1)
+    love.graphics.draw(image, x, y, 0, scale, scale)
+  end
+
+  function love.mousepressed(x, y, button)
+    if button == 1 then pointerDown = input("down", x, y) end
+  end
+  function love.mousereleased(x, y, button)
+    if button == 1 and pointerDown then
+      if not input("up", x, y) then send("I", "\ncancel,0,0") end
+      pointerDown = false
+    end
+  end
+  function love.touchpressed(_, x, y) input("down", x, y) end
+  function love.touchreleased(_, x, y) input("up", x, y) end
+  function love.keypressed(key)
+    if key == "escape" then love.event.quit() end
+  end
+  function love.quit()
+    if not commandedQuit then send("C") end
+    pcall(peer.disconnect_now, peer)
+  end
+end
+
+return DesktopCompanion
diff --git a/src/render/DesktopScreen.lua b/src/render/DesktopScreen.lua
new file mode 100644
index 00000000..8026bb80
--- /dev/null
+++ b/src/render/DesktopScreen.lua
@@ -0,0 +1,137 @@
+-- Cross-platform desktop secondary display.  LOVE owns one window, so a
+-- second minimal instance of this same app owns the companion window.  ENet
+-- is bundled with LOVE; binding it to loopback keeps frames and input local.
+
+local Platform = require("src.core.Platform")
+local HostShell = require("src.core.HostShell")
+local okEnet, enet = pcall(require, "enet")
+
+local DesktopScreen = {}
+local state = {
+  enabled = false, blocked = false, host = nil, peer = nil,
+  token = nil, port = nil, touches = {}, retryAt = 0, heartbeatAt = 0,
+}
+
+local function now()
+  return love and love.timer and love.timer.getTime and love.timer.getTime()
+    or os.clock()
+end
+
+local function destroy(sendQuit)
+  if state.peer and sendQuit then
+    pcall(state.peer.send, state.peer, "Q" .. state.token, 1, "reliable")
+  end
+  if state.peer then pcall(state.peer.disconnect_now, state.peer) end
+  if state.host then pcall(state.host.destroy, state.host) end
+  state.host, state.peer, state.token, state.port = nil, nil, nil, nil
+  state.touches = {}
+end
+
+local function token()
+  local seed = table.concat({ tostring(os.time()), tostring(now()), tostring({}) }, ":")
+  local digest = love.data.hash("sha256", seed)
+  return love.data.encode("string", "hex", digest):sub(1, 24)
+end
+
+local function start()
+  if state.host or state.blocked or now() < state.retryAt then
+    return state.host ~= nil
+  end
+  local base = 49152 + math.floor(now() * 1000) % 12000
+  for attempt = 0, 31 do
+    local port = 49152 + (base - 49152 + attempt * 37) % 12000
+    local ok, host = pcall(enet.host_create,
+      ("127.0.0.1:%d"):format(port), 1, 2)
+    if ok and host then
+      state.host, state.port, state.token = host, port, token()
+      local launched = HostShell.spawnSelfDetached({
+        ("--display-companion=%d,%s"):format(port, state.token),
+      })
+      if launched then return true end
+      destroy(false)
+      break
+    end
+  end
+  state.retryAt = now() + 1
+  return false
+end
+
+local function service()
+  if not state.enabled or state.blocked then return end
+  if not state.host and not start() then return end
+  while state.host do
+    local ok, event = pcall(state.host.service, state.host, 0)
+    if not ok then
+      destroy(false)
+      state.retryAt = now() + 1
+      return
+    end
+    if not event then break end
+    if event.type == "receive" then
+      local data = event.data or ""
+      if data == "H" .. state.token then
+        state.peer = event.peer
+      elseif event.peer == state.peer
+          and data:sub(1, #state.token + 2) == "I" .. state.token .. "\n" then
+        state.touches[#state.touches + 1] = data:sub(#state.token + 3)
+      elseif event.peer == state.peer and data == "C" .. state.token then
+        state.blocked = true
+        destroy(false)
+        return
+      end
+    elseif event.type == "disconnect" and event.peer == state.peer then
+      destroy(false)
+      state.retryAt = now() + 1
+      return
+    end
+  end
+  if state.peer and now() >= state.heartbeatAt then
+    state.heartbeatAt = now() + 1
+    pcall(state.peer.send, state.peer, "P" .. state.token, 1, "unreliable")
+  end
+end
+
+function DesktopScreen.usable()
+  return okEnet and enet ~= nil and Platform.canSpawnProcess()
+end
+
+function DesktopScreen.available()
+  return DesktopScreen.detected()
+end
+
+function DesktopScreen.detected()
+  service()
+  return state.peer ~= nil
+end
+
+function DesktopScreen.push(imageData, w, h, background, preference)
+  service()
+  if not state.peer or not imageData or not imageData.getString then return false end
+  w, h = tonumber(w), tonumber(h)
+  if not w or not h or w < 1 or h < 1 or w > 4096 or h > 4096 then return false end
+  local ok, raw = pcall(imageData.getString, imageData)
+  if not ok or type(raw) ~= "string" or #raw ~= w * h * 4 then return false end
+  local packed = love.data.compress("string", "lz4", raw, 1)
+  local header = ("F%s\n%d,%d,%u,%s\n"):format(state.token, w, h,
+    tonumber(background) or 0, tostring(preference or "auto"):gsub("[^%w_:.-]", ""))
+  local sent = pcall(state.peer.send, state.peer, header .. packed, 0, "reliable")
+  return sent
+end
+
+function DesktopScreen.pollTouch()
+  service()
+  return table.remove(state.touches, 1)
+end
+
+function DesktopScreen.setEnabled(on)
+  on = on == true
+  if on == state.enabled then
+    if on then service() end
+    return
+  end
+  state.enabled = on
+  state.blocked = false
+  if on then start(); service() else destroy(true) end
+end
+
+return DesktopScreen
diff --git a/src/render/SecondScreen.lua b/src/render/SecondScreen.lua
index 2e5fb3b5..d3b03796 100644
--- a/src/render/SecondScreen.lua
+++ b/src/render/SecondScreen.lua
@@ -1,11 +1,11 @@
--- Bridge to native secondary-display output (Android Presentation). The C
--- functions live in mobile/android/love/src/jni/love/src/common/android.cpp.
--- Everything is guarded: off Android, or if the symbols cannot be resolved,
--- this stays inert and the renderer keeps the in-window stacked layout.
+-- Shared secondary-display facade. Android uses its native Presentation
+-- bridge; process-capable desktop hosts fall back to a companion window.
+-- Everything is guarded, so unsupported hosts keep the in-window layout.
 
 local SecondScreen = {}
 local C = nil
 local ffi = nil
+local desktop = nil
 
 local function log(msg)
   pcall(function() require("src.core.Logger").info("SecondScreen: %s", msg) end)
@@ -37,17 +37,36 @@ do
   end
 end
 
+if not C then
+  local ok, backend = pcall(require, "src.render.DesktopScreen")
+  if ok and backend and backend.usable and backend.usable() then
+    desktop = backend
+    log("desktop companion backend ready")
+  end
+end
+
 function SecondScreen.usable()
-  return C ~= nil
+  return C ~= nil or desktop ~= nil
 end
 
 function SecondScreen.available()
+  if desktop then return desktop.available() end
   if not C then return false end
   local ok, r = pcall(C.love_android_secondary_ready)
   return ok and r ~= 0
 end
 
-function SecondScreen.push(imageData, w, h)
+-- A connected display is not necessarily the current Presentation yet. This
+-- distinction lets a companion retry its first frame after hotplug/re-target.
+function SecondScreen.detected()
+  if desktop then return desktop.detected() end
+  return SecondScreen.available()
+end
+
+function SecondScreen.push(imageData, w, h, background, preference)
+  if desktop then
+    return desktop.push(imageData, w, h, background, preference)
+  end
   if not C or not imageData then return false end
   return pcall(function()
     C.love_android_push_secondary(imageData:getFFIPointer(), w, h)
@@ -57,6 +76,7 @@ end
 -- Returns the oldest queued secondary-display event as "action,x,y", where
 -- coordinates are in the submitted frame's pixel space.
 function SecondScreen.pollTouch()
+  if desktop then return desktop.pollTouch() end
   if not C then return nil end
   local ok, event = pcall(function()
     return C.love_android_poll_secondary_touch()
@@ -66,6 +86,7 @@ function SecondScreen.pollTouch()
 end
 
 function SecondScreen.setEnabled(on)
+  if desktop then return desktop.setEnabled(on) end
   if not C then return end
   pcall(function() C.love_android_secondary_enable(on and 1 or 0) end)
 end
diff --git a/tests/engine/desktop_companion.lua b/tests/engine/desktop_companion.lua
new file mode 100644
index 00000000..36d8c5e1
--- /dev/null
+++ b/tests/engine/desktop_companion.lua
@@ -0,0 +1,56 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local clock, quit = 1, false
+local sent, queue, draws = {}, {}, 0
+local peer = {
+  send = function(_, data) sent[#sent + 1] = data end,
+  disconnect_now = function() end,
+}
+local host = {
+  connect = function() return peer end,
+  service = function()
+    if #queue == 0 then return nil end
+    return table.remove(queue, 1)
+  end,
+}
+local rendered = {
+  setFilter = function() end, replacePixels = function() end,
+  release = function() end,
+}
+love = {
+  timer = { getTime = function() return clock end },
+  data = { decompress = function(_, _, value) return value end },
+  image = { newImageData = function(w, h, format, raw)
+    assert(w == 1 and h == 1 and format == "rgba8" and raw == "rgba")
+    return {}
+  end },
+  graphics = {
+    newImage = function() return rendered end,
+    getDimensions = function() return 100, 100 end,
+    clear = function() end, setColor = function() end,
+    draw = function() draws = draws + 1 end,
+  },
+  event = { quit = function() quit = true end },
+}
+package.preload.enet = function()
+  return { host_create = function() return host end }
+end
+
+require("src.render.DesktopCompanion").install({ port = 50000, token = "token" })
+queue[#queue + 1] = { type = "connect" }
+love.update()
+assert(sent[#sent] == "Htoken", "companion authenticates after connecting")
+queue[#queue + 1] = {
+  type = "receive", data = "Ftoken\n1,1,0,auto\nrgba",
+}
+love.update()
+love.draw()
+assert(draws == 1, "companion draws a received frame")
+love.mousepressed(50, 50, 1)
+love.mousereleased(50, 50, 1)
+assert(sent[#sent - 1] == "Itoken\ndown,0,0"
+  and sent[#sent] == "Itoken\nup,0,0", "mouse input maps back to source pixels")
+queue[#queue + 1] = { type = "receive", data = "Qtoken" }
+love.update()
+assert(quit, "parent can close the companion")
+print("desktop companion: ok")
diff --git a/tests/engine/desktop_second_screen.lua b/tests/engine/desktop_second_screen.lua
new file mode 100644
index 00000000..77116a8b
--- /dev/null
+++ b/tests/engine/desktop_second_screen.lua
@@ -0,0 +1,57 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local clock = 1
+love = {
+  timer = { getTime = function() return clock end },
+  data = {
+    hash = function() return "digest" end,
+    encode = function() return "0123456789abcdef0123456789abcdef" end,
+    compress = function(_, _, value) return value end,
+  },
+}
+
+local sent, spawned, queue = {}, nil, {}
+local peer = {
+  send = function(_, data, channel, flag)
+    sent[#sent + 1] = { data = data, channel = channel, flag = flag }
+  end,
+  disconnect_now = function() end,
+}
+local host = {
+  service = function()
+    if #queue == 0 then return nil end
+    return table.remove(queue, 1)
+  end,
+  destroy = function() end,
+}
+
+package.loaded["src.core.Platform"] = { canSpawnProcess = function() return true end }
+package.loaded["src.core.HostShell"] = {
+  spawnSelfDetached = function(args) spawned = args return true end,
+}
+package.preload.enet = function()
+  return { host_create = function() return host end }
+end
+
+local Screen = require("src.render.SecondScreen")
+assert(Screen.usable(), "the shared facade selects the desktop backend")
+Screen.setEnabled(true)
+assert(spawned and spawned[1]:match("^%-%-display%-companion=%d+,[%w]+$"),
+  "enabling launches one companion of this app")
+local token = spawned[1]:match(",([%w]+)$")
+queue[#queue + 1] = { type = "receive", peer = peer, data = "H" .. token }
+assert(Screen.detected(), "a token-authenticated companion becomes detected")
+
+local pixels = { getString = function() return "rgba" end }
+assert(Screen.push(pixels, 1, 1, 0x102030, "auto"),
+  "a connected companion accepts a frame")
+assert(sent[#sent].data:find("^F" .. token .. "\n1,1,1056816,auto\nrgba"),
+  "frame metadata and pixels stay in one loopback packet")
+
+queue[#queue + 1] = {
+  type = "receive", peer = peer, data = "I" .. token .. "\ndown,3,4",
+}
+assert(Screen.pollTouch() == "down,3,4", "companion input returns to the mod")
+Screen.setEnabled(false)
+assert(sent[#sent].data == "Q" .. token, "disabling closes the companion")
+print("desktop second screen: ok")
diff --git a/tests/engine/hostshell_spawn_self.lua b/tests/engine/hostshell_spawn_self.lua
new file mode 100644
index 00000000..e3f2b937
--- /dev/null
+++ b/tests/engine/hostshell_spawn_self.lua
@@ -0,0 +1,32 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local osName, fused, command = "Windows", true, nil
+love = {
+  system = { getOS = function() return osName end },
+  filesystem = {
+    getExecutablePath = function() return "C:\\Game\\gen1recomp.exe" end,
+    getSource = function() return "C:\\Source\\gen1recomp" end,
+    isFused = function() return fused end,
+  },
+}
+package.loaded["src.core.Platform"] = { canSpawnProcess = function() return true end }
+local execute = os.execute
+os.execute = function(value) command = value return 0 end
+
+local HostShell = require("src.core.HostShell")
+assert(HostShell.spawnSelfDetached({ "--display-companion=50000,token" }))
+assert(command:find('start "" /b ', 1, true)
+  and command:find('"C:\\Game\\gen1recomp.exe"', 1, true),
+  "Windows launches the fused app detached")
+
+osName, fused = "Linux", false
+assert(HostShell.spawnSelfDetached({ "--display-companion=50000,token" }))
+assert(command:find("'C:\\Source\\gen1recomp'", 1, true)
+  and command:sub(-1) == "&", "Linux source runs include the game folder")
+
+osName, fused = "OS X", true
+assert(HostShell.spawnSelfDetached({ "--display-companion=50000,token" }))
+assert(not command:find("start", 1, true) and command:sub(-1) == "&",
+  "macOS uses the same detached POSIX path")
+os.execute = execute
+print("spawn self detached: ok")