Currently, with texture paint (both UV and 3D), large brushes or quick brush movements can draw on nearby, yet unconnected, UV islands. This is especially an annoyance when in 3D texture paint, you can accidentally paint over parts of your model that are geometrically distant from the location you are currently painting over.
I'd like an option where selected UV faces can act as a mask, allowing texture paint to only affect the image underneath those faces in the UV map.