Merge pull request #5736 from AvaloniaUI/extended-client-area-win32-maintain-size

maintain the client size when switching between extended and non exte…

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -483,8 +483,8 @@ namespace Avalonia.Win32
                     IntPtr.Zero,
                     0,
                     0,
-                    requestedClientWidth + (windowRect.Width - clientRect.Width),
-                    requestedClientHeight + (windowRect.Height - clientRect.Height),
+                    requestedClientWidth + (_isClientAreaExtended ? 0 : windowRect.Width - clientRect.Width),
+                    requestedClientHeight + (_isClientAreaExtended ? 0 : windowRect.Height - clientRect.Height),
                     SetWindowPosFlags.SWP_RESIZE);
             }
         }
@@ -883,20 +883,19 @@ namespace Avalonia.Win32
                 _isClientAreaExtended = false;
                 return;
             }
-
-            GetWindowRect(_hwnd, out var rcClient);
+            GetClientRect(_hwnd, out var rcClient);
+            GetWindowRect(_hwnd, out var rcWindow);
 
             // Inform the application of the frame change.
             SetWindowPos(_hwnd,
-                         IntPtr.Zero,
-                         rcClient.left, rcClient.top,
-                         rcClient.Width, rcClient.Height,
-                         SetWindowPosFlags.SWP_FRAMECHANGED);
+                IntPtr.Zero,
+                rcWindow.left, rcWindow.top,
+                rcClient.Width, rcClient.Height,
+                SetWindowPosFlags.SWP_FRAMECHANGED);
 
             if (_isClientAreaExtended && WindowState != WindowState.FullScreen)
             {
                 var margins = UpdateExtendMargins();
-
                 DwmExtendFrameIntoClientArea(_hwnd, ref margins);
             }
             else
@@ -906,6 +905,8 @@ namespace Avalonia.Win32
 
                 _offScreenMargin = new Thickness();
                 _extendedMargins = new Thickness();
+                
+                Resize(new Size(rcWindow.Width/ RenderScaling, rcWindow.Height / RenderScaling));
             }
 
             if(!_isClientAreaExtended || (_extendChromeHints.HasFlagCustom(ExtendClientAreaChromeHints.SystemChrome) &&