As a Roblox developer, it is currently impossible to make 9-sliced images that work well with UIScale.
Recently there was a new property added to ImageLabel and ImageButton named SliceScale. In the latest update (359), there was a change that made it only accessible to CoreScripts.
For developers using UIScale to scale their interface, SliceScale was crucial to making sure all slices were high quality on objects such as rounded rectangles when scaled. Before SliceScale, it was impossible to make a 9-sliced image higher resolution than what it was to be displayed at. Here’s a comparison of what a rounded rectangle (10px corners) looks like at 2x scaling with, and without a sufficient image resolution:
I’d like for this API to be non-CoreScript accessible, or for there to be an equivalent API added.