Abusing PDF field objects to render a monochrome grid, combined with keystroke-entry in a text field to receive inputs.

More info here: https://th0mas.nl/2025/01/12/tetris-in-a-pdf/

This works in PDFium (Chromium-based browsers) and PDF.js (Firefox). Not tested for / intended to function in other engines (but you might be able modify this to work in Acrobat I guess).