fix: Batman: Arkham City starts with full x87 precision under FEX (#792)

Arkham City (BatmanAC.exe) stopped with Unreal's "Fatal error!" about
1.5 s after start: an access violation in BatmanAC.exe right after
steam.dll loads. The game leans on x87 and needs full precision under
FEX (FEX wiki), while the default FEX preset sets
FEX_X87REDUCEDPRECISION=1. Engine fixes sit above the shared preset and
below the player's own game profile, so a fix keyed on BatmanAC.exe
beside the started exe (BatmanAC.exe or BmLauncher.exe) restores
x87 precision for this game only. Arkham Asylum, which shares
BmLauncher.exe but not BatmanAC.exe, keeps the preset.

tools/linuxfs/overlay/usr/local/bin/droiddeck-game-env

@@ -98,6 +98,14 @@ def godot_sentry(game):
         return {"WINEDLLOVERRIDES": ";".join(n + "=d" for n in sentry)}, [], "Godot: Sentry crash handler off"
 
 
+@fix
+def arkham_city_x87(game):
+    """Batman: Arkham City (BatmanAC.exe, also started through BmLauncher.exe beside it) leans on x87
+    and stops with Unreal's "Fatal error!" right after start when FEX reduces its precision."""
+    if "batmanac.exe" in game.files:
+        return {"FEX_X87REDUCEDPRECISION": "0"}, [], "Batman: Arkham City: FEX with full x87 precision"
+
+
 def skip_red_launcher(command):
     """Cyberpunk 2077's prelauncher opens REDlauncher, a Qt WebEngine window that hands over to the
     game half a minute later and then stays up beside it, well over a core busy for as long as the

tools/tests/test_game_environment.py

@@ -173,6 +173,25 @@ class GameEnvironmentTest(unittest.TestCase):
             (other / "Other.exe").touch()
             self.assertEqual(MODULE["engine_fixes"](str(other / "Other.exe"), []), ({}, [], []))
 
+    def test_arkham_city_gets_full_x87_precision_over_the_preset(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            city = Path(tmp) / "Batman Arkham City GOTY/Binaries/Win32"
+            city.mkdir(parents=True)
+            for name in ("BatmanAC.exe", "BmLauncher.exe"):
+                (city / name).touch()
+            for exe in ("BatmanAC.exe", "BmLauncher.exe"):
+                env, extra, notes = MODULE["engine_fixes"](str(city / exe), [])
+                self.assertEqual((env, extra), ({"FEX_X87REDUCEDPRECISION": "0"}, []))
+            config = {"version": 1, "shared": {"FEX_X87REDUCEDPRECISION": "1"}}
+            self.assertEqual(MODULE["apply_config"]({}, config, "200260", env)["FEX_X87REDUCEDPRECISION"], "0")
+            config["games"] = {"200260": {"FEX_X87REDUCEDPRECISION": "1"}}
+            self.assertEqual(MODULE["apply_config"]({}, config, "200260", env)["FEX_X87REDUCEDPRECISION"], "1")
+            asylum = Path(tmp) / "Batman Arkham Asylum GOTY/Binaries"
+            asylum.mkdir(parents=True)
+            for name in ("ShippingPC-BmGame.exe", "BmLauncher.exe"):
+                (asylum / name).touch()
+            self.assertEqual(MODULE["engine_fixes"](str(asylum / "BmLauncher.exe"), []), ({}, [], []))
+
     def test_a_new_fix_only_needs_registering(self):
         with tempfile.TemporaryDirectory() as tmp:
             (Path(tmp) / "Game.exe").touch()