Make ShowDialog respect WindowStartupLocation.

`SetWindowStartupLocation` needs to be called _after_ the window is shown as is done in `Show`. In addition, the owner window needs to be passed so we can get the correct screen.

Renamed the `parent` parameter to `ShowDialog` to `owner` as its `WindowStartupLocation.CenterOwner` not `CenterParent`.

src/Avalonia.Controls/Window.cs

@@ -394,39 +394,42 @@ namespace Avalonia.Controls
         /// <summary>
         /// Shows the window as a dialog.
         /// </summary>
+        /// <param name="owner">The dialog's owner window.</param>
         /// <returns>
         /// A task that can be used to track the lifetime of the dialog.
         /// </returns>
-        public Task ShowDialog(Window parent)
+        public Task ShowDialog(Window owner)
         {
-            return ShowDialog<object>(parent);
+            return ShowDialog<object>(owner);
         }
 
-
         /// <summary>
         /// Shows the window as a dialog.
         /// </summary>
         /// <typeparam name="TResult">
         /// The type of the result produced by the dialog.
         /// </typeparam>
+        /// <param name="owner">The dialog's owner window.</param>
         /// <returns>.
         /// A task that can be used to retrieve the result of the dialog when it closes.
         /// </returns>
-        public Task<TResult> ShowDialog<TResult>(Window parent) => ShowDialog<TResult>(parent.PlatformImpl);
-        
+        public Task<TResult> ShowDialog<TResult>(Window owner) => ShowDialog<TResult>(owner.PlatformImpl);
+
         /// <summary>
         /// Shows the window as a dialog.
         /// </summary>
         /// <typeparam name="TResult">
         /// The type of the result produced by the dialog.
         /// </typeparam>
+        /// <param name="owner">The dialog's owner window.</param>
         /// <returns>.
         /// A task that can be used to retrieve the result of the dialog when it closes.
         /// </returns>
-        public Task<TResult> ShowDialog<TResult>(IWindowImpl parent)
+        public Task<TResult> ShowDialog<TResult>(IWindowImpl owner)
         {
-            if(parent == null)
-                throw new ArgumentNullException(nameof(parent));
+            if(owner == null)
+                throw new ArgumentNullException(nameof(owner));
+
             if (IsVisible)
             {
                 throw new InvalidOperationException("The window is already being shown.");
@@ -435,15 +438,15 @@ namespace Avalonia.Controls
             AddWindow(this);
 
             EnsureInitialized();
-            SetWindowStartupLocation();
             IsVisible = true;
             LayoutManager.ExecuteInitialLayoutPass(this);
 
+            var result = new TaskCompletionSource<TResult>();
+
             using (BeginAutoSizing())
             {
 
-                PlatformImpl?.ShowDialog(parent);
-                var result = new TaskCompletionSource<TResult>();
+                PlatformImpl?.ShowDialog(owner);
 
                 Renderer?.Start();
                 Observable.FromEventPattern<EventHandler, EventArgs>(
@@ -452,17 +455,18 @@ namespace Avalonia.Controls
                     .Take(1)
                     .Subscribe(_ =>
                     {
-                        parent.Activate();
+                        owner.Activate();
                         result.SetResult((TResult)(_dialogResult ?? default(TResult)));
                     });
-
-                return result.Task;
             }
+
+            SetWindowStartupLocation(owner);
+            return result.Task;
         }
 
-        void SetWindowStartupLocation()
+        private void SetWindowStartupLocation(IWindowImpl owner = null)
         {
-            var scaling = PlatformImpl?.Scaling ?? 1;
+            var scaling = owner?.Scaling ?? PlatformImpl?.Scaling ?? 1;
 
             // TODO: We really need non-client size here.
             var rect = new PixelRect(
@@ -471,7 +475,7 @@ namespace Avalonia.Controls
 
             if (WindowStartupLocation == WindowStartupLocation.CenterScreen)
             {
-                var screen = Screens.ScreenFromPoint(Position);
+                var screen = Screens.ScreenFromPoint(owner?.Position ?? Position);
 
                 if (screen != null)
                 {
@@ -480,12 +484,12 @@ namespace Avalonia.Controls
             }
             else if (WindowStartupLocation == WindowStartupLocation.CenterOwner)
             {
-                if (Owner != null)
+                if (owner != null)
                 {
                     // TODO: We really need non-client size here.
                     var ownerRect = new PixelRect(
-                        Owner.Position,
-                        PixelSize.FromSize(Owner.ClientSize, scaling));
+                        owner.Position,
+                        PixelSize.FromSize(owner.ClientSize, scaling));
                     Position = ownerRect.CenterRect(rect).Position;
                 }
             }