fix(app): Windows keeps secrets out of captures by its real version

Before Windows 10 2004, SetWindowDisplayAffinity takes
WDA_EXCLUDEFROMCAPTURE without failing and acts as WDA_MONITOR, so the
fallback on a failure never ran and its comment was wrong (the short
reading). Both apps' runners now choose by the real version, RtlGetVersion
(GetVersionEx tells what the manifest claims): WDA_EXCLUDEFROMCAPTURE from
build 19041 on, WDA_MONITOR before. Checked for syntax only: nothing here
builds for Windows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/windows/runner/flutter_window.cpp

@@ -12,6 +12,35 @@
 #define WDA_EXCLUDEFROMCAPTURE 0x00000011
 #endif
 
+namespace {
+
+// Whether this Windows keeps a window out of captures with
+// WDA_EXCLUDEFROMCAPTURE: Windows 10 2004 (build 19041) and later. Before,
+// it takes the value without failing and acts as WDA_MONITOR, so the
+// version decides, not a failure. RtlGetVersion tells the real version;
+// GetVersionEx tells what the app's manifest claims.
+bool ExcludesFromCapture() {
+  using RtlGetVersionFunction = LONG(WINAPI*)(PRTL_OSVERSIONINFOW);
+  const HMODULE ntdll = GetModuleHandleW(L"ntdll.dll");
+  if (!ntdll) {
+    return false;
+  }
+  const auto rtl_get_version = reinterpret_cast<RtlGetVersionFunction>(
+      GetProcAddress(ntdll, "RtlGetVersion"));
+  if (!rtl_get_version) {
+    return false;
+  }
+  RTL_OSVERSIONINFOW version = {};
+  version.dwOSVersionInfoSize = sizeof(version);
+  if (rtl_get_version(&version) != 0) {
+    return false;
+  }
+  return version.dwMajorVersion > 10 ||
+         (version.dwMajorVersion == 10 && version.dwBuildNumber >= 19041);
+}
+
+}  // namespace
+
 FlutterWindow::FlutterWindow(const flutter::DartProject& project)
     : project_(project) {}
 
@@ -60,11 +89,11 @@ void FlutterWindow::GuardScreen() {
                  result) {
         if (call.method_name() == "hold") {
           secrets_shown_++;
-          // Before Windows 10 2004 the exclusion fails: the window shows
-          // black in captures instead.
-          if (!SetWindowDisplayAffinity(GetHandle(), WDA_EXCLUDEFROMCAPTURE)) {
-            SetWindowDisplayAffinity(GetHandle(), WDA_MONITOR);
-          }
+          // Before Windows 10 2004 the window shows black in captures
+          // instead.
+          static const bool excludes = ExcludesFromCapture();
+          SetWindowDisplayAffinity(
+              GetHandle(), excludes ? WDA_EXCLUDEFROMCAPTURE : WDA_MONITOR);
           result->Success();
         } else if (call.method_name() == "release") {
           secrets_shown_ = secrets_shown_ > 0 ? secrets_shown_ - 1 : 0;