[HostsFileEditor]Fix hosts content dialog on top of title bar (#21924)

* [Hosts]Don't overlap title bar with dialogs

* Use VisualTreeHelper instead

src/modules/Hosts/Hosts/Helpers/VisualTreeUtils.cs

@@ -0,0 +1,67 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Media;
+
+namespace Hosts.Helpers
+{
+    // Taken from https://github.com/microsoft/microsoft-ui-xaml/blob/main/test/MUXControlsTestApp/Utilities/VisualTreeUtils.cs
+    // Original copyright header:
+    // Copyright (c) Microsoft Corporation. All rights reserved.
+    // Licensed under the MIT License. See LICENSE in the project root for license information.
+    public static class VisualTreeUtils
+    {
+        public static T FindVisualChildByType<T>(this DependencyObject element)
+            where T : DependencyObject
+        {
+            if (element == null)
+            {
+                return null;
+            }
+
+            if (element is T elementAsT)
+            {
+                return elementAsT;
+            }
+
+            int childrenCount = VisualTreeHelper.GetChildrenCount(element);
+            for (int i = 0; i < childrenCount; i++)
+            {
+                var result = VisualTreeHelper.GetChild(element, i).FindVisualChildByType<T>();
+                if (result != null)
+                {
+                    return result;
+                }
+            }
+
+            return null;
+        }
+
+        public static FrameworkElement FindVisualChildByName(this DependencyObject element, string name)
+        {
+            if (element == null || string.IsNullOrWhiteSpace(name))
+            {
+                return null;
+            }
+
+            if (element is FrameworkElement elementAsFE && elementAsFE.Name == name)
+            {
+                return elementAsFE;
+            }
+
+            int childrenCount = VisualTreeHelper.GetChildrenCount(element);
+            for (int i = 0; i < childrenCount; i++)
+            {
+                var result = VisualTreeHelper.GetChild(element, i).FindVisualChildByName(name);
+                if (result != null)
+                {
+                    return result;
+                }
+            }
+
+            return null;
+        }
+    }
+}

src/modules/Hosts/Hosts/Views/MainPage.xaml

@@ -323,6 +323,7 @@
         <ContentDialog
             x:Name="EntryDialog"
             x:Uid="EntryDialog"
+            Loaded="ContentDialog_Loaded_ApplyMargin"
             IsPrimaryButtonEnabled="{Binding Valid, Mode=TwoWay}"
             PrimaryButtonStyle="{StaticResource AccentButtonStyle}">
             <ContentDialog.DataContext>
@@ -368,6 +369,7 @@
         <ContentDialog
             x:Name="AdditionalLinesDialog"
             x:Uid="AdditionalLinesDialog"
+            Loaded="ContentDialog_Loaded_ApplyMargin"
             PrimaryButtonCommand="{x:Bind UpdateAdditionalLinesCommand}"
             PrimaryButtonStyle="{StaticResource AccentButtonStyle}">
 

src/modules/Hosts/Hosts/Views/MainPage.xaml.cs

@@ -6,6 +6,7 @@ using System;
 using System.Threading.Tasks;
 using System.Windows.Input;
 using CommunityToolkit.Mvvm.Input;
+using Hosts.Helpers;
 using Hosts.Models;
 using Hosts.Settings;
 using Hosts.ViewModels;
@@ -181,5 +182,19 @@ namespace Hosts.Views
                 }
             }
         }
+
+        private void ContentDialog_Loaded_ApplyMargin(object sender, RoutedEventArgs e)
+        {
+            try
+            {
+                // Based on the template from dev/CommonStyles/ContentDialog_themeresources.xaml in https://github.com/microsoft/microsoft-ui-xaml
+                var border = Helpers.VisualTreeUtils.FindVisualChildByName(sender as ContentDialog, "BackgroundElement") as Border;
+                border.Margin = new Thickness(0, 32, 0, 0); // Should be the size reserved for the title bar as in MainWindow.xaml
+            }
+            catch (Exception ex)
+            {
+                Logger.LogError("Couldn't set the margin for a content dialog. It will appear on top of the title bar.", ex);
+            }
+        }
     }
 }