In the original HyperCard, if you picked a pattern (say, bricks) and filled an area, then picked another pattern (say diagonal lines) and clicked in the same place, only one brick would fill with diagonal lines: the pattern is converted to black-and-white pixels on the canvas, and that limits the next use of the flood tool.
In Decker, filling with diagonal lines replaces all the area that was filled with bricks, not just one brick.
That means the drawing tools aren’t drawing black-and-white pixels, they’re drawing pattern numbers, and sixty times a second Decker converts those pattern numbers into patterns of black-and-white pixels. Animated patterns would work by just changing which pattern to use on every consecutive frame.
As a result, the flood-fill tool works how childhood-me expected it to, and things like canvas.segment[] do the right thing with tiled dither patterns, instead of messing them up. However, it might also mean that “coloured patterns” are more difficult than you’d expect.