Create a blocking/modal dialog with a new window on UWP. No dependencies.

UWP Modal Dialog.cs

private async void Settings_Click(object sender, PointerRoutedEventArgs e)
{
    var mainViewId = ApplicationView.GetForCurrentView().Id;

    CoreApplicationView newView = CoreApplication.CreateNewView();
    int newViewId = 0;
    var popupClosed = new TaskCompletionSource<bool>();

    Window settingsWindow = null;
    var mainWindow = Window.Current;
    bool settingsClosing = false;
    async void FocusSettingsDialog(object sender, dynamic @event)
    {
        if (!settingsClosing)
        {
            await ApplicationViewSwitcher.SwitchAsync(newViewId, mainViewId, ApplicationViewSwitchingOptions.SkipAnimation);
        }
    }
    mainWindow.Activated += FocusSettingsDialog;

    await newView.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
    {
        Frame frame = new Frame();
        frame.Navigate(typeof(SettingsDialog), null);
        settingsWindow = Window.Current;
        Window.Current.Content = frame;
        var page = (SettingsDialog)frame.Content;
        // You have to activate the window in order to show it later.
        Window.Current.Activate();


        newViewId = ApplicationView.GetForCurrentView().Id;

        var settingsDialog = (SettingsDialog)frame.Content;
        settingsDialog.Closing += async () =>
        {
            settingsClosing = true;
            await mainWindow.Dispatcher.RunAsync(CoreDispatcherPriority.High, () => mainWindow.Activated -= FocusSettingsDialog);
            popupClosed.TrySetResult(true);
        };
        settingsDialog.CancelClicked += async () =>
        {
            await ApplicationViewSwitcher.SwitchAsync(mainViewId, newViewId, ApplicationViewSwitchingOptions.ConsolidateViews);
        };
        settingsDialog.SaveClicked += async () =>
        {
            await ApplicationViewSwitcher.SwitchAsync(mainViewId, newViewId, ApplicationViewSwitchingOptions.ConsolidateViews);
            // Save settings here
            await settingsDialog.Settings.SaveAsync();
            await App.ReloadSettingsAsync();
        };

        settingsDialog.Settings = App.Settings;
    });

    bool viewShown = await ApplicationViewSwitcher.TryShowAsStandaloneAsync(newViewId);
    await newView.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
    {
        var frame = (Frame)Window.Current.Content;
        var page = (SettingsDialog)frame.Content;
        var resized = ApplicationView.GetForCurrentView().TryResizeView(new Windows.Foundation.Size(page.ActualWidth, page.ActualHeight));
    });

    await popupClosed.Task;
    await ApplicationViewSwitcher.SwitchAsync(mainViewId);
}