Merge pull request #9695 from AvaloniaUI/rounded_corner

Windows - Set rounded corners when client is extended

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

@@ -820,6 +820,14 @@ namespace Avalonia.Win32.Interop
             DWMWA_LAST
         };
 
+        public enum DwmWindowCornerPreference : uint
+        {
+            DWMWCP_DEFAULT = 0,
+            DWMWCP_DONOTROUND,
+            DWMWCP_ROUND,
+            DWMWCP_ROUNDSMALL
+        }
+
         public enum MapVirtualKeyMapTypes : uint
         {
             MAPVK_VK_TO_VSC = 0x00,

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -1029,6 +1029,12 @@ namespace Avalonia.Win32
             {
                 var margins = UpdateExtendMargins();
                 DwmExtendFrameIntoClientArea(_hwnd, ref margins);
+
+                unsafe
+                {
+                    int cornerPreference = (int)DwmWindowCornerPreference.DWMWCP_ROUND;
+                    DwmSetWindowAttribute(_hwnd, (int)DwmWindowAttribute.DWMWA_WINDOW_CORNER_PREFERENCE, &cornerPreference, sizeof(int));
+                }
             }
             else
             {
@@ -1039,6 +1045,12 @@ namespace Avalonia.Win32
                 _extendedMargins = new Thickness();
 
                 Resize(new Size(rcWindow.Width / RenderScaling, rcWindow.Height / RenderScaling), PlatformResizeReason.Layout);
+
+                unsafe
+                {
+                    int cornerPreference = (int)DwmWindowCornerPreference.DWMWCP_DEFAULT;
+                    DwmSetWindowAttribute(_hwnd, (int)DwmWindowAttribute.DWMWA_WINDOW_CORNER_PREFERENCE, &cornerPreference, sizeof(int));
+                }
             }
 
             if (!_isClientAreaExtended || (_extendChromeHints.HasAllFlags(ExtendClientAreaChromeHints.SystemChrome) &&