[example] Simple-tetris example is glitchy

I made the simple-tetris example a bit too quickly, would like to polish at "some point"™
It's made to be an example on a simple game. The polish isn't super important.

Start by reviewing the simple-tetris example and determine which glitches or polish issues are intended to be addressed. The issue does not name a file, test, or specific behavior, so first establish a concrete scope and a way to verify that the example is improved.

Written by the indexing model from the issue text.