A UI frame at any size without deforming its corners: the panel, the button and the dialogue box of every game. The top row stretches it as a sprite and the corners smear.
The other three use createNineSlice: slice: 8 cuts it into nine parts, the corners stay as drawn, and mode says how the edges and the middle fill their space ('stretch', 'tile' or 'tile-fit'). Resizing is changing width; scaleX would stretch the corners too.