After the fix still causing issues, I figured the value must be >0xffff_ffff so not fitting. I changed to logic to handle that case too.

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -1877,7 +1877,10 @@ namespace Avalonia.Win32.Interop
 
         public static uint LGID(IntPtr HKL)
         {
-            return (uint)(HKL.ToInt64() & 0xffff);
+            unchecked
+            {
+                return (uint)((ulong)HKL & 0xffff);
+            }
         }
 
         public const int SORT_DEFAULT = 0;