Paint

The app every desktop used to ship with, for the times you need to draw a box round something, block out a name in a screenshot, or make a quick diagram.

The flood fill is a scanline implementation, so filling a large region does not overflow the stack the way a naive recursive fill does. Undo replays a stroke list rather than storing bitmaps, which keeps it exact and cheap, and the canvas stays crisp on a high-DPI screen.