diff --git a/docs/building.md b/docs/building.md
index db8220d0..cc2f30bd 100644
--- a/docs/building.md
+++ b/docs/building.md
@@ -444,6 +444,9 @@ Before a commit:
 - `python tools/check_content.py --staged`: nothing game-derived and no compiled shader is staged. CI runs the same check;
   `git config core.hooksPath .githooks` runs it before each local commit.
 - `build\<ver>\nx2navi.exe --selftest`: layout, dispatch table and host-function registry, without game files.
+- `python tools\check_platform.py` after a runtime change outside `runtime/platform/`: no shared file uses Win32 outside
+  a branch that only a Windows build compiles (`#ifdef _WIN32`, the `#else` of `#ifndef _WIN32`).
+  `python tests\check_platform_test.py` checks the scanner itself on made-up files.
 
 A change to the in-game menu's look (`runtime/ui/menu_skin.cpp`, `option_rows.cpp`, `perf_panel.cpp`) can be looked at
 without game files: `python tools/menu_preview.py` builds the menu's own drawing code with the host's clang++ and
diff --git a/tests/check_platform_test.py b/tests/check_platform_test.py
new file mode 100644
index 00000000..c217b6d9
--- /dev/null
+++ b/tests/check_platform_test.py
@@ -0,0 +1,70 @@
+"""tools/check_platform.py on made-up runtime/ trees: a Win32 use behind a Windows-only preprocessor branch passes, an
+unguarded one fails, and the repository's own runtime/ passes. No game files; stdlib only.
+
+Run: python tests/check_platform_test.py
+"""
+import contextlib
+import io
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+from unittest import mock
+
+ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(ROOT / 'tools'))
+
+import check_platform  # noqa: E402
+
+
+class CheckPlatformTest(unittest.TestCase):
+    def check(self, files, runtime=None):
+        """(exit code, output) of check_platform.py on a runtime/ folder holding files ({name: text}), or on
+        runtime."""
+        with tempfile.TemporaryDirectory() as d:
+            if runtime is None:
+                runtime = Path(d) / 'runtime'
+                runtime.mkdir()
+                for name, text in files.items():
+                    (runtime / name).write_text(text, encoding='utf-8')
+            out = io.StringIO()
+            with mock.patch.object(sys, 'argv', ['check_platform.py', '--runtime', str(runtime)]), \
+                    contextlib.redirect_stdout(out):
+                code = check_platform.main()
+        return code, out.getvalue()
+
+    def test_windows_only_branches_pass(self):
+        code, out = self.check({
+            'a.c': '#ifdef _WIN32\n#include <windows.h>\n#endif\nint a;\n',
+            'b.c': '#ifndef _WIN32\n#include <dlfcn.h>\nvoid f(int h) { (void)h; }\n#else\n'
+                   'void f(HANDLE h) { CloseHandle(h); }\n#endif\n',
+            'c.cpp': '#if defined(_WIN32)   // the DLL\n#  if X\nstatic DWORD d;\n#  endif\n'
+                     'static void g() { FreeLibrary(0); }\n#elif Y\nint y;\n#else\nint z;\n#endif\n',
+            'd.h': '#if !defined(_WIN32)\nint p;\n#elif Z\nint q;\n#else\n#include <windows.h>\n#endif\n',
+            'e.c': '/* CloseHandle in a comment */\nconst char *s = "GetLastError";\n',
+        })
+        self.assertEqual(code, 0, out)
+        self.assertNotIn('FAIL', out)
+
+    def test_unguarded_uses_fail(self):
+        code, out = self.check({
+            'a.c': '#include <windows.h>\nint a;\n',
+            'b.c': '#ifndef _WIN32\nint b;\n#endif\nvoid f(void *h) { CloseHandle(h); }\n',
+            'c.c': '#ifdef _WIN32\nint c;\n#else\nstatic DWORD d;\n#endif\n',
+            'd.c': '#ifdef _WIN32\nint d;\n#elif 1\nvoid g(void) { Sleep(1); }\n#endif\n',
+            'e.c': '#if defined(_WIN32) && X\nvoid h(void) { Sleep(1); }\n#endif\n',   # scanned: never let through
+            'f.c': '#ifdef _WIN32\n#endif\n\n#include <windows.h>\n',
+            'ok.c': '#ifdef _WIN32\nvoid f(void *h) { CloseHandle(h); }\n#endif\n',
+        })
+        self.assertEqual(code, 1)
+        for name in 'abcdef':
+            self.assertIn(f'FAIL runtime/{name}.c', out)
+        self.assertNotIn('runtime/ok.c', out)
+
+    def test_the_runtime_passes(self):
+        code, out = self.check({}, ROOT / 'runtime')
+        self.assertEqual(code, 0, out)
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)
diff --git a/tools/check_platform.py b/tools/check_platform.py
index b74a2b76..cbd40667 100644
--- a/tools/check_platform.py
+++ b/tools/check_platform.py
@@ -3,12 +3,15 @@
 
 Runtime code goes through runtime/platform/platform.h. The Win32 back ends of the window system are Win32 by design and
 are not scanned: files named *_win.c / *_win.cpp (runtime/ and runtime/ui/), whose SDL counterparts (*_sdl.*) serve
-other hosts. Any other file that still uses Win32 goes into ALLOWED with the reason; a file in ALLOWED that no longer
+other hosts. Nor are the branches of a shared file that only a Windows build compiles (windows_only_lines): a file
+with a portable path of its own may reach a Windows-only library there (the upscalers' DLLs, RivaTuner's shared
+memory). Any other file that still uses Win32 goes into ALLOWED with the reason; a file in ALLOWED that no longer
 matches is noted, so the entry gets removed.
 
 Usage: python tools/check_platform.py [--root DIR]   (DIR holds runtime/; default: the repository root)
 """
 import argparse
+import bisect
 import re
 import sys
 from pathlib import Path
@@ -54,11 +57,49 @@ def strip(text):
     return re.sub(r'"(?:\\.|[^"\\\n])*"', blank, text)
 
 
+DIRECTIVE = re.compile(r"#\s*(if|ifdef|ifndef|elif|else|endif)\b")
+WIN_IF = re.compile(r"#\s*(ifdef\s+_WIN32|if\s+defined\s*\(\s*_WIN32\s*\)|if\s+defined\s+_WIN32|if\s+_WIN32)$")
+WIN_IFNOT = re.compile(r"#\s*(ifndef\s+_WIN32|if\s+!\s*defined\s*\(\s*_WIN32\s*\)|if\s+!\s*defined\s+_WIN32)$")
+
+
+def windows_only_lines(code):
+    """The numbers (from 0) of the lines of code (comments blanked: strip) that only a Windows build compiles: the
+    branch of #ifdef _WIN32, #if defined(_WIN32) or #if _WIN32 up to its #else, #elif or #endif, the #else branch of
+    #ifndef _WIN32 or #if !defined(_WIN32), and whatever a conditional inside such a branch holds. The directives
+    themselves are not among them. Any other branch counts as compiled everywhere, #elif defined(_WIN32) and
+    #if defined(_WIN32) && X too: such a branch is scanned, which can fail the check but never lets a use through."""
+    lines, stack = set(), []   # stack: [kind ('win', 'notwin', 'other'), whether its open branch is Windows-only]
+    for n, line in enumerate(code.split("\n")):
+        s = line.strip()
+        m = DIRECTIVE.match(s)
+        if not m:
+            if any(w for _, w in stack):
+                lines.add(n)
+            continue
+        d = m.group(1)
+        if d in ("if", "ifdef", "ifndef"):
+            kind = "win" if WIN_IF.match(s) else "notwin" if WIN_IFNOT.match(s) else "other"
+            stack.append([kind, kind == "win"])
+        elif d == "else" and stack:
+            stack[-1][1] = stack[-1][0] == "notwin"
+        elif d == "elif" and stack:
+            stack[-1][1] = False
+        elif d == "endif" and stack:
+            stack.pop()
+    return lines
+
+
 def scan(path):
     raw = path.read_text(encoding="utf-8", errors="replace")
-    hits = [("include", m.group(0).strip()) for m in INCLUDE.finditer(raw)]
-    code = strip(raw)
-    hits += [("call", m.group(1)) for m in CALL_RE.finditer(code)]
+    code = strip(raw)   # the same lines as raw
+    skip = windows_only_lines(code)
+    starts = [0] + [i + 1 for i, c in enumerate(raw) if c == "\n"]
+
+    def guarded(pos):
+        return bisect.bisect_right(starts, pos) - 1 in skip
+    # an include's match can begin on the line before it (^\s* takes that line's newline): its end is on its own line
+    hits = [("include", m.group(0).strip()) for m in INCLUDE.finditer(raw) if not guarded(m.end() - 1)]
+    hits += [("call", m.group(1)) for m in CALL_RE.finditer(code) if not guarded(m.start())]
     return hits