What is RepaintBoundary and how does it optimize painting?

RepaintBoundary creates a separate display list (layer) for its children, isolating repaints.

If a specific widget (like a ticking clock) updates frequently, it forces its parent and siblings to repaint. By wrapping the clock in a RepaintBoundary, Flutter paints the clock into its own layer. The rest of the screen is painted once, and the GPU just composites the updated clock layer over the static background.