Use the View > Other Windows > Xamarin. Forms Previewer menu in Visual Studio to open the preview window.
How do I enable Xamarin previewer?
You can turn the XAML Previewer off in the Tools > Options > Xamarin > Xamarin. Forms XAML Previewer dialog by selecting Default XML Editor as your Default XAML Editor.
How do I open XAML live preview?
The XAML Live Preview window is available during debugging. To open it, go to Debug > Windows > XAML Live Preview. Or, select the Show in XAML Live Preview button in the application toolbar.