Material vs locked: the rule that lets 2048 and Sudoku share a board

I've always loved Sudoku and 2048. They're the puzzles I open instead of scrolling. For a long time I wanted a mashup that felt like both, not a gimmick with two logos stuck on one grid. Play Number Merge Sudoku (Daily or Endless). What follows is the design decision that made it playable. Why mashups usually fail Sudoku says: every digit appears...