The Stretch field is a member of the Sandbox.UI.BorderImageRepeat enumeration. It specifies that the edge regions of a source image should be stretched to fill the gap between each border. This is one of the possible values for the border-image-repeat CSS property, which is used to define how the border image is repeated or stretched.
Use BorderImageRepeat.Stretch when you want the border image's edges to be stretched to fit the border area without repeating. This is useful when you want a seamless look without visible tiling.
// Example of using BorderImageRepeat.Stretch in a UI component
var myPanel = new Panel();
myPanel.Style.BorderImage = "url('path/to/image.png') 30 stretch";
myPanel.Style.BorderImageRepeat = BorderImageRepeat.Stretch;