I am trying to get smooth corners. I have tried a variety of images. I have tried 9 slicing. How can I fix this?
At this time, you'll have to enable MSAA to achieve smooth, antialiased corners. This is in the quality settings (Edit > Project Settings > Quality > Anti Aliasing). Also make sure that your camera has MSAA enabled. Note that we may provide additional antialiasing methods in the future.
My bad, I forgot an important detail. The runtime panel currently renders in as an overlay after the cameras are done rendering. This means that MSAA settings are ignored by the UIElements runtime panel, they only works in an editor context right now. Since we only provide the overlay mode at this time, you options are limited. I think your only option is to set a RenderTexture with antialiasing enabled to your panel, then draw that texture over your scene. Again, we will provide more convenient options in the future.