Some improvements for smudge + hints + resources

You can toggle between smudge "liquify" (alt key) and "smooth" to blend colors, smooth details, distort reflections. Useful to create caricatures, water simulations and other interesting effects.

Comparing to others, Pencil2D's liquify is based on the original algorythm, but it's not the original, in order to be easily accessible and efficient to work with; it doesn't require an extra window. 

Regarding non-pixel-destructiveness and accuracy, future improvements are possible but they seem not urgent, not even important.