Design windows in the Window painter, which has several views where you specify how a window looks and behaves.
The default layout for the Window painter workspace has two views:
This view has buttons that let you split the Design and XAML panes either vertically or horizontally, and allows you to hide one of the panes. You can also swap the pane positions by clicking the double arrow icon between the Design and XAML panes.
You can also use the context menu in the Solution Explorer to open only one of these tabs.