Add hackfix for win32 bug. (#16143)

Not sure I can describe it any better than the in-code comment.

Co-authored-by: Max Katz <maxkatz6@outlook.com>

src/Windows/Avalonia.Win32/WindowImpl.AppWndProc.cs

@@ -130,6 +130,7 @@ namespace Avalonia.Win32
                     }
 
                 case WindowsMessage.WM_DPICHANGED:
+                    if (!_ignoreDpiChanges)
                     {
                         _dpi = (uint)wParam >> 16;
                         var newDisplayRect = Marshal.PtrToStructure<RECT>(lParam);
@@ -151,6 +152,7 @@ namespace Avalonia.Win32
 
                         return IntPtr.Zero;
                     }
+                    break;
 
                 case WindowsMessage.WM_GETICON:
                     if (_iconImpl == null)

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -107,6 +107,7 @@ namespace Avalonia.Win32
         private bool _shown;
         private bool _hiddenWindowIsParent;
         private uint _langid;
+        private bool _ignoreDpiChanges;
         internal bool _ignoreWmChar;
         private WindowTransparencyLevel _transparencyLevel;
         private readonly WindowTransparencyLevel _defaultTransparencyLevel;
@@ -707,7 +708,20 @@ namespace Avalonia.Win32
 
             _hiddenWindowIsParent = parentHwnd == OffscreenParentWindow.Handle;
 
+            // I can't find mention of this *anywhere* online, but it seems that setting
+            // GWL_HWNDPARENT to a window which is on the non-primary monitor can cause two
+            // WM_DPICHANGED messages to be sent: the first changing the DPI to the parent's DPI,
+            // then another changing the DPI back. This then causes Windows to provide an incorrect
+            // suggested new rectangle to the WM_DPICHANGED message if the window is immediately
+            // moved to the parent window's monitor (e.g. when using
+            // WindowStartupLocation.CenterOwner) causing the window to be shown with an incorrect
+            // size.
+            //
+            // Just ignore any WM_DPICHANGED while we're setting the parent as this shouldn't
+            // change the DPI anyway.
+            _ignoreDpiChanges = true;
             SetWindowLongPtr(_hwnd, (int)WindowLongParam.GWL_HWNDPARENT, parentHwnd);
+            _ignoreDpiChanges = false;
 
             // Windows doesn't seem to respect the HWND_TOPMOST flag of a window when showing an owned window for the first time.
             // So we set the HWND_TOPMOST again before the owned window is shown. This only needs to be done once.