When enabling opacity on win10, automatically enable acrylic

   In #11180 we made `opacity` independent from `useAcrylic`. We also changed the mouse wheel behavior to only change opacity, and not mess with acrylic.

   However, on Windows 10, vintage opacity doesn't work at all. So there, we still need to manually enable acrylic when the user requests opacity.

   * [x] Closes #11285

src/cascadia/TerminalControl/ControlCore.cpp

@@ -35,6 +35,22 @@ constexpr const auto UpdatePatternLocationsInterval = std::chrono::milliseconds(
 
 namespace winrt::Microsoft::Terminal::Control::implementation
 {
+    // Helper to check if we're on Windows 11 or not. This is used to check if
+    // we need to use acrylic to achieve transparency, because vintage opacity
+    // doesn't work in islands on win10.
+    // Remove when we can remove the rest of GH#11285
+    static bool _isVintageOpacityAvailable() noexcept
+    {
+        OSVERSIONINFOEXW osver{};
+        osver.dwOSVersionInfoSize = sizeof(osver);
+        osver.dwBuildNumber = 22000; // TODO! switch me to 22000
+
+        DWORDLONG dwlConditionMask = 0;
+        VER_SET_CONDITION(dwlConditionMask, VER_BUILDNUMBER, VER_GREATER_EQUAL);
+
+        return VerifyVersionInfoW(&osver, VER_BUILDNUMBER, dwlConditionMask) != FALSE;
+    }
+
     // Helper static function to ensure that all ambiguous-width glyphs are reported as narrow.
     // See microsoft/terminal#2066 for more info.
     static bool _IsGlyphWideForceNarrowFallback(const std::wstring_view /* glyph */)
@@ -444,6 +460,24 @@ namespace winrt::Microsoft::Terminal::Control::implementation
 
         _settings.Opacity(newOpacity);
 
+        // GH#11285 - If the user is on Windows 10, and they changed the
+        // transparency of the control s.t. it should be partially opaque, then
+        // opt them in to acrylic. It's the only way to have transparency on
+        // Windows 10.
+        // We'll also turn the acrylic back off when they're fully opaque, which
+        // is what the Terminal did prior to 1.12.
+        if (!_isVintageOpacityAvailable())
+        {
+            if (newOpacity < 1.0)
+            {
+                _settings.UseAcrylic(true);
+            }
+            else
+            {
+                _settings.UseAcrylic(false);
+            }
+        }
+
         auto eventArgs = winrt::make_self<TransparencyChangedEventArgs>(newOpacity);
         _TransparencyChangedHandlers(*this, *eventArgs);
     }
@@ -572,6 +606,14 @@ namespace winrt::Microsoft::Terminal::Control::implementation
 
         _settings = settings;
 
+        // GH#11285 - If the user is on Windows 10, and they wanted opacity, but
+        // didn't explicitly request acrylic, then opt them in to acrylic.
+        // On Windows 11+, this isn't needed, because we can have vintage opacity.
+        if (!_isVintageOpacityAvailable() && _settings.Opacity() < 1.0 && !_settings.UseAcrylic())
+        {
+            _settings.UseAcrylic(true);
+        }
+
         // Initialize our font information.
         const auto fontFace = _settings.FontFace();
         const short fontHeight = ::base::saturated_cast<short>(_settings.FontSize());