diff --git a/base/filesystem/file_util.h b/base/filesystem/file_util.h
index ca17903d..83f605e2 100644
--- a/base/filesystem/file_util.h
+++ b/base/filesystem/file_util.h
@@ -26,6 +26,13 @@
 
 #include <base/arch.h>
 
+// <windows.h> maps CreateDirectory to CreateDirectoryW. Declare and call the
+// base function under its own name whether or not a Windows header came first,
+// as environment_variables.h does for its names.
+#if defined(CreateDirectory)
+#undef CreateDirectory
+#endif
+
 namespace base {
 
 // Create a directory within another directory.
diff --git a/base/filesystem/platform_file.h b/base/filesystem/platform_file.h
index 8bb25552..b663b7ca 100644
--- a/base/filesystem/platform_file.h
+++ b/base/filesystem/platform_file.h
@@ -7,7 +7,9 @@
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
-#include <windows.h>
+// minwin.h, not <windows.h>: this header reaches every user of base::File, and
+// <windows.h>'s macros (near, far, MemoryBarrier, ...) must not follow it there.
+#include "base/win/minwin.h"
 #include "base/win/scoped_handle.h"
 #endif
 
diff --git a/base/filesystem/win/file_util_win.cc b/base/filesystem/win/file_util_win.cc
index 67a4f3df..86c0bf53 100644
--- a/base/filesystem/win/file_util_win.cc
+++ b/base/filesystem/win/file_util_win.cc
@@ -9,6 +9,9 @@
 #include "base/strings/format.h"
 
 #include <windows.h>
+// This file defines base::CreateDirectory; <windows.h> just mapped the name to
+// CreateDirectoryW. The Win32 call below is spelled CreateDirectoryW directly.
+#undef CreateDirectory
 #include <base/threading/scoped_blocking_call.h>
 
 namespace base {
diff --git a/base/filesystem/win/memory_mapped_file_win.cc b/base/filesystem/win/memory_mapped_file_win.cc
index 2152ebeb..e1178104 100644
--- a/base/filesystem/win/memory_mapped_file_win.cc
+++ b/base/filesystem/win/memory_mapped_file_win.cc
@@ -4,6 +4,8 @@
 #include <base/logging.h>
 #include <base/filesystem/memory_mapped_file.h>
 
+#include <windows.h>
+
 namespace base {
 
 bool MemoryMappedFile::Map() {
diff --git a/base/threading/mutex.h b/base/threading/mutex.h
index 2b2b1c09..0505cd06 100644
--- a/base/threading/mutex.h
+++ b/base/threading/mutex.h
@@ -28,8 +28,6 @@ using SharedMutex = std::shared_mutex;
 
 #if defined(__linux__) || defined(OS_LINUX) || defined(OS_POSIX)
 #include <pthread.h>
-#elif defined(_WIN32) || defined(OS_WIN)
-#include <base/win/minwin.h>
 #endif
 
 namespace base {
@@ -60,24 +58,22 @@ class SharedMutex {
 
 #elif defined(_WIN32) || defined(OS_WIN)
 
-class SharedMutex {
+// Out of line (mutex_win.cc) so this header never pulls a Windows header and
+// its macros into the including file.
+class BASE_EXPORT SharedMutex {
  public:
   constexpr SharedMutex() noexcept = default;
 
   SharedMutex(const SharedMutex&) = delete;
   SharedMutex& operator=(const SharedMutex&) = delete;
 
-  void lock() { ::AcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_)); }
-  void unlock() { ::ReleaseSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_)); }
-  bool try_lock() {
-    return !!::TryAcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
-  }
-
-  void lock_shared() { ::AcquireSRWLockShared(reinterpret_cast<PSRWLOCK>(&lock_)); }
-  void unlock_shared() { ::ReleaseSRWLockShared(reinterpret_cast<PSRWLOCK>(&lock_)); }
-  bool try_lock_shared() {
-    return !!::TryAcquireSRWLockShared(reinterpret_cast<PSRWLOCK>(&lock_));
-  }
+  void lock();
+  void unlock();
+  bool try_lock();
+
+  void lock_shared();
+  void unlock_shared();
+  bool try_lock_shared();
 
  private:
   // Matches the SRWLOCK ABI (single void*). SRWLOCK_INIT is {nullptr}.
diff --git a/base/threading/mutex_test.cc b/base/threading/mutex_test.cc
new file mode 100644
index 00000000..c2c033af
--- /dev/null
+++ b/base/threading/mutex_test.cc
@@ -0,0 +1,80 @@
+// Copyright (C) 2026 Vincent Hengel.
+// For licensing information see LICENSE at the root of this distribution.
+
+// These headers come first, before anything else can pull in <windows.h>: they
+// reach nearly every file and must not bring its macros along, or names like
+// MemoryBarrier or near get rewritten under them.
+#include <base/threading/mutex.h>
+#include <base/threading/spinning_mutex.h>
+
+#if defined(MemoryBarrier) || defined(near) || defined(far) || defined(min) || defined(max)
+#error "a base lock header leaked a Windows macro"
+#endif
+
+// base::File needs HANDLE, which it takes from minwin.h. On mingw minwin.h is
+// all of <windows.h>, so only the other compilers can hold it to this.
+#include <base/filesystem/file.h>
+
+#if !defined(__MINGW32__) && \
+    (defined(MemoryBarrier) || defined(near) || defined(far) || defined(min) || defined(max))
+#error "base/filesystem/file.h leaked a Windows macro"
+#endif
+
+#include <gtest/gtest.h>
+
+#include <base/atomic.h>
+#include <base/threading/lock_guard.h>
+#include <base/threading/thread.h>
+
+namespace {
+
+TEST(SpinningMutex, TryFailsWhileHeld) {
+  base::SpinningMutex mutex;
+  mutex.lock();
+  EXPECT_FALSE(mutex.try_lock());
+  mutex.unlock();
+  EXPECT_TRUE(mutex.try_lock());
+  mutex.unlock();
+}
+
+TEST(SpinningMutex, SerializesIncrements) {
+  constexpr int kThreads = 4;
+  constexpr int kIterations = 20000;
+  base::SpinningMutex mutex;
+  int counter = 0;
+  base::Thread* threads[kThreads];
+  for (auto& t : threads) {
+    t = new base::Thread(
+        "mutex-inc",
+        [&] {
+          for (int i = 0; i < kIterations; i++) {
+            base::LockGuard<base::SpinningMutex> guard(mutex);
+            ++counter;
+          }
+        },
+        /*start_now=*/true);
+  }
+  for (auto* t : threads) {
+    EXPECT_TRUE(t->Join());
+    delete t;
+  }
+  EXPECT_EQ(counter, kThreads * kIterations);
+}
+
+TEST(SharedMutex, ReadersShareWritersExclude) {
+  base::SharedMutex mutex;
+  mutex.lock_shared();
+  EXPECT_TRUE(mutex.try_lock_shared());
+  EXPECT_FALSE(mutex.try_lock());
+  mutex.unlock_shared();
+  mutex.unlock_shared();
+
+  mutex.lock();
+  EXPECT_FALSE(mutex.try_lock_shared());
+  EXPECT_FALSE(mutex.try_lock());
+  mutex.unlock();
+  EXPECT_TRUE(mutex.try_lock());
+  mutex.unlock();
+}
+
+}  // namespace
diff --git a/base/threading/mutex_win.cc b/base/threading/mutex_win.cc
new file mode 100644
index 00000000..af584cea
--- /dev/null
+++ b/base/threading/mutex_win.cc
@@ -0,0 +1,42 @@
+// Copyright (C) 2026 Vincent Hengel.
+// For licensing information see LICENSE at the root of this distribution.
+//
+// SharedMutex on SRWLOCK. Out of line rather than inline in mutex.h so that
+// minwin.h, which on mingw is all of <windows.h>, stays out of every file that
+// merely holds a mutex: its macros (MemoryBarrier, near, far, ...) would
+// otherwise rewrite unrelated names there.
+
+#include <base/threading/mutex.h>
+#include <base/win/minwin.h>
+
+namespace base {
+
+#if !defined(BASE_USE_STD_MUTEX) || !BASE_USE_STD_MUTEX
+
+void SharedMutex::lock() {
+  ::AcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+
+void SharedMutex::unlock() {
+  ::ReleaseSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+
+bool SharedMutex::try_lock() {
+  return !!::TryAcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+
+void SharedMutex::lock_shared() {
+  ::AcquireSRWLockShared(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+
+void SharedMutex::unlock_shared() {
+  ::ReleaseSRWLockShared(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+
+bool SharedMutex::try_lock_shared() {
+  return !!::TryAcquireSRWLockShared(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+
+#endif
+
+}  // namespace base
diff --git a/base/threading/spinning_mutex.cc b/base/threading/spinning_mutex.cc
index 980269ee..6aed3cc2 100644
--- a/base/threading/spinning_mutex.cc
+++ b/base/threading/spinning_mutex.cc
@@ -7,6 +7,7 @@
 
 #if defined(_WIN32) || defined(OS_WIN)
 #include <intrin.h>
+#include <base/win/minwin.h>
 #define PA_YIELD_PROCESSOR _mm_pause()
 #elif defined(__x86_64__) || defined(__i386__)
 #define PA_YIELD_PROCESSOR __asm__ __volatile__("pause")
@@ -25,6 +26,22 @@
 
 namespace base {
 
+#if defined(_WIN32) || defined(OS_WIN)
+// Out of line on Windows so spinning_mutex.h never needs minwin.h, which on
+// mingw is all of <windows.h>. lock_ has SRWLOCK's layout, a single pointer.
+void SpinningMutex::LockSlow() {
+  ::AcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+
+bool SpinningMutex::Try() {
+  return !!::TryAcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+
+void SpinningMutex::Release() {
+  ::ReleaseSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
+}
+#endif
+
 void SpinningMutex::Reinit() {
   Release();
 }
diff --git a/base/threading/spinning_mutex.h b/base/threading/spinning_mutex.h
index 23409b59..12436b41 100644
--- a/base/threading/spinning_mutex.h
+++ b/base/threading/spinning_mutex.h
@@ -11,8 +11,6 @@
 #include <errno.h>
 #include <pthread.h>
 #include <base/atomic.h>
-#elif defined(_WIN32) || defined(OS_WIN)
-#include <base/win/minwin.h>
 #endif
 
 namespace base {
@@ -21,8 +19,11 @@ class BASE_EXPORT SpinningMutex {
  public:
   inline constexpr SpinningMutex();
   STRONG_INLINE void Acquire();
-  STRONG_INLINE void Release();
-  STRONG_INLINE bool Try();
+  // Inline on POSIX; out of line on Windows (spinning_mutex.cc), so that
+  // this header, which every base::Mutex user includes, never pulls a Windows
+  // header and its macros into the including file.
+  void Release();
+  bool Try();
   void AssertAcquired() const {}
   void Reinit();
 
@@ -50,10 +51,8 @@ class BASE_EXPORT SpinningMutex {
 
   base::Atomic<int32_t> state_{kUnlocked};
 #elif defined(_WIN32) || defined(OS_WIN)
-  struct PA_CHROME_SRWLOCK {
-    void* Ptr;
-  };
-  PA_CHROME_SRWLOCK lock_{SRWLOCK_INIT};
+  // SRWLOCK's layout, a single pointer; SRWLOCK_INIT is all zero.
+  void* lock_ = nullptr;
 #endif
 };
 
@@ -83,19 +82,4 @@ STRONG_INLINE void SpinningMutex::Release() {
 }
 #endif
 
-#if defined(_WIN32) || defined(OS_WIN)
-
-STRONG_INLINE void SpinningMutex::LockSlow() {
-  ::AcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
-}
-
-STRONG_INLINE bool SpinningMutex::Try() {
-  return !!::TryAcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
-}
-
-STRONG_INLINE void SpinningMutex::Release() {
-  ::ReleaseSRWLockExclusive(reinterpret_cast<PSRWLOCK>(&lock_));
-}
-#endif
-
 }  // namespace base