Couldn't the rendering still be batched? In Swing, for example, onDraw is invoked whenever Swing feels like it (and so could presumably be batched) regardless of when and how often the component's state is updated.
All the drawing in QWidgets is done imperatively with QPainter. Converting immediate mode painting to retained mode painting is possible, I guess... but not easy and probably full of pitfalls. Also, performance is just fine (really good actually) in practice, at least on PC-class hardware. QPainter's backend is not quite as fast as Skia, but still fast.
