Add Window IsDialog Property (#19226)

Co-authored-by: Sebastian Redinger <sebastian.redinger@chaos.com>
Co-authored-by: Max Katz <maxkatz6@outlook.com>

src/Avalonia.Controls/Window.cs

@@ -481,6 +481,11 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Gets whether this window was opened as a dialog
+        /// </summary>
+        public bool IsDialog => _showingAsDialog;
+
         /// <summary>
         /// Starts moving a window with left button being held. Should be called from left mouse button press event handler
         /// </summary>
@@ -761,12 +766,12 @@ namespace Avalonia.Controls
                     return null;
                 }
 
+                _showingAsDialog = modal;
                 RaiseEvent(new RoutedEventArgs(WindowOpenedEvent));
 
                 EnsureInitialized();
                 ApplyStyling();
                 _shown = true;
-                _showingAsDialog = modal;
                 IsVisible = true;
 
                 // If window position was not set before then platform may provide incorrect scaling at this time,
@@ -984,7 +989,7 @@ namespace Avalonia.Controls
             {
                 return;
             }
-            
+
             var location = GetEffectiveWindowStartupLocation(owner);
 
             switch (location)
@@ -1017,7 +1022,7 @@ namespace Avalonia.Controls
 
             return startupLocation;
         }
-        
+
         private void SetWindowStartupLocation(Window? owner = null)
         {
             if (_wasShownBefore)
@@ -1052,7 +1057,7 @@ namespace Avalonia.Controls
 
                 screen ??= Screens.ScreenFromPoint(Position);
                 screen ??= Screens.Primary;
-                
+
                 if (screen is not null)
                 {
                     var childRect = screen.WorkingArea.CenterRect(rect);
@@ -1072,7 +1077,7 @@ namespace Avalonia.Controls
                 var childRect = ownerRect.CenterRect(rect);
 
                 var screen = Screens.ScreenFromWindow(owner);
-                
+
                 childRect = ApplyScreenConstraint(screen, childRect);
 
                 Position = childRect.Position;
@@ -1080,7 +1085,7 @@ namespace Avalonia.Controls
 
             if (!_positionWasSet && DesktopScaling != PlatformImpl?.DesktopScaling) // Platform returns incorrect scaling, forcing setting position may fix it
                 PlatformImpl?.Move(Position);
-            
+
             PixelRect ApplyScreenConstraint(Screen? screen, PixelRect childRect)
             {
                 if (screen?.WorkingArea is { } constraint)