Window.SetTitleBar(null) does not restore the default drag region.

[nandin-borjigin]

Describe the bug

As per the documentation, calling SetTitleBar(null) should reset the drag region to default. However, it does so only if ExtendsContentIntoTitleBar is toggled after calling it.

Steps to reproduce the bug

<?xml version="1.0" encoding="utf-8" ?>
<Window
    x:Class="App1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="using:App1"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Title="App1"
    mc:Ignorable="d">

    <Window.SystemBackdrop>
        <MicaBackdrop />
    </Window.SystemBackdrop>
    <Border>
        <StackPanel
            HorizontalAlignment="Center"
            VerticalAlignment="Center"
            Orientation="Vertical">
            <TextBlock
                x:Name="Output"
                HorizontalAlignment="Center"
                Text="Default" />
            <StackPanel
                HorizontalAlignment="Center"
                Orientation="Horizontal"
                Spacing="12">
                <Button
                    x:Name="Restore"
                    Click="Restore_Click"
                    Content="Restore" />
                <Button
                    x:Name="Custom"
                    Click="Custom_Click"
                    Content="Custom" />
            </StackPanel>
        </StackPanel>
    </Border>
</Window>

using Microsoft.UI.Xaml;
// To learn more about WinUI, the WinUI project structure,
// and more about our project templates, see: http://aka.ms/winui-project-info.

namespace App1
{
    /// <summary>
    /// An empty window that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            ExtendsContentIntoTitleBar = true;
        }

        private void Restore_Click(object sender, RoutedEventArgs e)
        {
            Output.Text = "Default";
            SetTitleBar(null);
           
            // It wouldn't work unless the following lines are uncommented.
            // i.e., the `Content` still responds to drag after executing the above lines.
            // ExtendsContentIntoTitleBar = false;
            // ExtendsContentIntoTitleBar = true;
        }

        private void Custom_Click(object sender, RoutedEventArgs e)
        {
            Output.Text = "Custom";
            SetTitleBar(Content);
        }
    }
}

Expected behavior

SetTitleBar(null) should restore the default drag region.

Screenshots

Video.mp4

NuGet package version

WinUI 3 - Windows App SDK 1.7.3: 1.7.250606001

Windows version

Windows 11 (24H2): Build 26100

Additional context

No response