enable drop shadow on win32 popups.

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

@@ -909,8 +909,17 @@ namespace Avalonia.Win32.Interop
 
         public enum ClassLongIndex : int
         {
-            GCL_HCURSOR = -12,
-            GCL_HICON = -14
+            GCLP_MENUNAME = -8,
+            GCLP_HBRBACKGROUND = -10,
+            GCLP_HCURSOR = -12,
+            GCLP_HICON = -14,
+            GCLP_HMODULE = -16,
+            GCL_CBWNDEXTRA = -18,
+            GCL_CBCLSEXTRA = -20,
+            GCLP_WNDPROC = -24,
+            GCL_STYLE = -26,
+            GCLP_HICONSM = -34,
+            GCW_ATOM = -32
         }
 
         [DllImport("user32.dll", EntryPoint = "SetClassLongPtr")]
@@ -929,6 +938,20 @@ namespace Avalonia.Win32.Interop
             return SetClassLong64(hWnd, nIndex, dwNewLong);
         }
 
+        public static IntPtr GetClassLongPtr(IntPtr hWnd, int nIndex)
+        {
+            if (IntPtr.Size > 4)
+                return GetClassLongPtr64(hWnd, nIndex);
+            else
+                return new IntPtr(GetClassLongPtr32(hWnd, nIndex));
+        }
+
+        [DllImport("user32.dll", EntryPoint = "GetClassLong")]
+        public static extern uint GetClassLongPtr32(IntPtr hWnd, int nIndex);
+
+        [DllImport("user32.dll", EntryPoint = "GetClassLongPtr")]
+        public static extern IntPtr GetClassLongPtr64(IntPtr hWnd, int nIndex);
+
         [DllImport("user32.dll", EntryPoint = "SetCursor")]
         internal static extern IntPtr SetCursor(IntPtr hCursor);
 

src/Windows/Avalonia.Win32/PopupImpl.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Win32
                 UnmanagedMethods.WindowStyles.WS_EX_TOOLWINDOW |
                 UnmanagedMethods.WindowStyles.WS_EX_TOPMOST;
 
-            return UnmanagedMethods.CreateWindowEx(
+            var result = UnmanagedMethods.CreateWindowEx(
                 (int)exStyle,
                 atom,
                 null,
@@ -37,6 +37,14 @@ namespace Avalonia.Win32
                 IntPtr.Zero,
                 IntPtr.Zero,
                 IntPtr.Zero);
+
+            var classes = (int)UnmanagedMethods.GetClassLongPtr(result, (int)UnmanagedMethods.ClassLongIndex.GCL_STYLE);
+
+            classes |= (int)UnmanagedMethods.ClassStyles.CS_DROPSHADOW;
+
+            UnmanagedMethods.SetClassLong(result, UnmanagedMethods.ClassLongIndex.GCL_STYLE, new IntPtr(classes));
+
+            return result;
         }
 
         protected override IntPtr WndProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam)

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -384,7 +384,7 @@ namespace Avalonia.Win32
         public void SetCursor(IPlatformHandle cursor)
         {
             var hCursor = cursor?.Handle ?? DefaultCursor;
-            UnmanagedMethods.SetClassLong(_hwnd, UnmanagedMethods.ClassLongIndex.GCL_HCURSOR, hCursor);
+            UnmanagedMethods.SetClassLong(_hwnd, UnmanagedMethods.ClassLongIndex.GCLP_HCURSOR, hCursor);
 
             if (_owner.IsPointerOver)
                 UnmanagedMethods.SetCursor(hCursor);