The object keeps sliding on its background surface as long as at least one of its pixels is visible, unless it encounters a collision that cannot be resolved otherwise. Once that's no longer the case, it pops forward (keeping the same spot under your mouse) until it is.
There are 2 other videos where hopefully you can see this happening. Or, better yet, you can try it running it yourself on Windows or Mac.
There are 2 other videos where hopefully you can see this happening. Or, better yet, you can try it running it yourself on Windows or Mac.