diff --git a/app/src/main/assets/linuxfs/usr/local/bin/winnative-session b/app/src/main/assets/linuxfs/usr/local/bin/winnative-session
index 19496286f..691dd50d2 100644
--- a/app/src/main/assets/linuxfs/usr/local/bin/winnative-session
+++ b/app/src/main/assets/linuxfs/usr/local/bin/winnative-session
@@ -50,7 +50,8 @@ if [ -z "${WN_INSIDE:-}" ]; then
   elif [ "${WN_REFRESH:-0}" -gt 0 ]; then
     args+=(-r "$WN_REFRESH")
   fi
-  [ "$mode" = steam ] && args+=(-e)
+  # Keep game windows at the session size when Steam returns focus on Resume.
+  [ "$mode" = steam ] && args+=(-e --force-windows-fullscreen)
   export WN_INSIDE=1
   exec gamescope "${args[@]}" -- "$0" "$mode" "$@"
 fi
diff --git a/tools/linuxfs/overlay/usr/local/bin/winnative-session b/tools/linuxfs/overlay/usr/local/bin/winnative-session
index 19496286f..691dd50d2 100755
--- a/tools/linuxfs/overlay/usr/local/bin/winnative-session
+++ b/tools/linuxfs/overlay/usr/local/bin/winnative-session
@@ -50,7 +50,8 @@ if [ -z "${WN_INSIDE:-}" ]; then
   elif [ "${WN_REFRESH:-0}" -gt 0 ]; then
     args+=(-r "$WN_REFRESH")
   fi
-  [ "$mode" = steam ] && args+=(-e)
+  # Keep game windows at the session size when Steam returns focus on Resume.
+  [ "$mode" = steam ] && args+=(-e --force-windows-fullscreen)
   export WN_INSIDE=1
   exec gamescope "${args[@]}" -- "$0" "$mode" "$@"
 fi
diff --git a/tools/linuxfs/tests/test_session.py b/tools/linuxfs/tests/test_session.py
new file mode 100644
index 000000000..20b37e255
--- /dev/null
+++ b/tools/linuxfs/tests/test_session.py
@@ -0,0 +1,74 @@
+import json
+from pathlib import Path
+import subprocess
+import tempfile
+import unittest
+
+
+ROOT = Path(__file__).resolve().parents[3]
+SESSION = ROOT / 'app/src/main/assets/linuxfs/usr/local/bin/winnative-session'
+
+
+class SessionTests(unittest.TestCase):
+    def setUp(self):
+        self.temp = tempfile.TemporaryDirectory()
+        self.addCleanup(self.temp.cleanup)
+        self.root = Path(self.temp.name)
+        self.bin = self.root / 'bin'
+        self.bin.mkdir()
+        self.capture = self.root / 'launch.json'
+        self.env = {
+            'PATH': str(self.bin),
+            'HOME': str(self.root),
+            'WN_CAPTURE': str(self.capture),
+        }
+        for name in ('mkdir', 'seq'):
+            (self.bin / name).symlink_to('/usr/bin/' + name)
+        self.stub('wayland-info', '#!/bin/sh\nexit 0\n')
+        self.stub('gamescope', '''#!/usr/bin/python3
+import json
+import os
+import sys
+with open(os.environ['WN_CAPTURE'], 'w') as output:
+    json.dump({'args': sys.argv[1:], 'inside': os.environ.get('WN_INSIDE')}, output)
+''')
+
+    def stub(self, name, contents):
+        path = self.bin / name
+        path.write_text(contents)
+        path.chmod(0o755)
+
+    def launch(self, *args, **env):
+        subprocess.run(['/bin/bash', str(SESSION), *args],
+                       env={**self.env, **env}, check=True, capture_output=True, timeout=5)
+        return json.loads(self.capture.read_text())
+
+    def test_steam_enforces_session_size_for_game_windows(self):
+        launch = self.launch('steam', 'steam://rungameid/123',
+                             WN_WIDTH='1920', WN_HEIGHT='1080', WN_REFRESH='120')
+        self.assertEqual(launch['args'], [
+            '--backend', 'wayland', '--expose-wayland', '-f',
+            '-W', '1920', '-H', '1080', '-w', '1920', '-h', '1080', '-r', '120',
+            '-e', '--force-windows-fullscreen', '--', str(SESSION),
+            'steam', 'steam://rungameid/123',
+        ])
+        self.assertEqual(launch['inside'], '1')
+
+    def test_frame_limit_takes_precedence_over_refresh_rate(self):
+        args = self.launch('steam', WN_FPS='40', WN_REFRESH='120')['args']
+        self.assertEqual(args[args.index('-r') + 1], '40')
+        self.assertIn('--force-windows-fullscreen', args)
+
+    def test_other_modes_keep_their_window_policy(self):
+        for mode, target in (('run', '/games/My Game/game.sh'), ('desktop', '')):
+            with self.subTest(mode=mode):
+                args = self.launch(mode, *([target] if target else []))['args']
+                self.assertNotIn('--force-windows-fullscreen', args)
+                self.assertNotIn('-e', args)
+                self.assertNotIn('-r', args)
+                self.assertEqual(args[args.index('--') + 1:],
+                                 [str(SESSION), mode] + ([target] if target else []))
+
+
+if __name__ == '__main__':
+    unittest.main()