ScrollViewer

The ScrollViewer has two properties that govern the visibility of the vertical and horizontal scroll bars. By default, the VerticalScrollBarVisibility property is the enumeration member ScrollBarVisibility.Visible, which means that the scroll bar is always visible, but it's inactive when it's not needed.

The default setting of the HorizontalScrollBarVisibility property is ScrollBarVisibility.Disabled, which means that the scroll bar is never visible.

Another option is ScrollBarVisibility.Auto, which means that the scroll bar appears only when it's needed. You might try setting the VerticalScrollBar property to this member and also decrease the number of buttons to something that can fit on your screen. As you make the window tall enough for the vertical scroll bar to disappear, you'll notice that all the buttons get wider because they have additional space in which to stretch! (Of course, you can avoid that effect by setting the HorizontalAlignment property of the buttons to HorizontalAlignment.Center.)

You can set the HorizontalScrollBarVisibility property to ScrollBarVisibility.Visible or ScrollBarVisibility.Auto. Now when you make the window too narrow for the buttons, a horizontal scroll bar lets you scroll the right sides into view. Without a horizontal scroll bar, when you make the window narrow, the ScrollViewer gives the StackPanel a narrower width. A very narrow StackPanel makes the buttons less wide, and button content is truncated. With a horizontal scroll bar, the StackPanel gets the width it needs.