It will not let you throw the board away.
Take matching tiles off the board, two at a time, until none are left. That is the whole game, and it is a hundred years older than this page.
Every version of this game you have ever played can be lost in a way that is not your fault to notice: around move twenty you take a pair that quietly makes the board impossible, and then you play on for another half hour before it tells you. We measured it — on a shuffled board, a careless player strands it 56.5% of the time, and then keeps playing for a median of 22 more moves after the game is already over.
That cannot happen here. A match that would leave the board unfinishable is simply not played. The board says so, says why, and stays exactly as it was. So every position you are ever looking at can still be cleared, which means there is nothing to undo and no reason for an undo button.
Not being stopped. Clearing a board without the guardrail ever stepping in is called clean, and it is the only thing in this game you can do badly — which is why it is the only thing kept as a record. For what it is worth, the trap is rare and decisive: of the positions we measured, under one in ten contained a fatal move at all. You can play a whole board and never meet one. You can also walk straight into the one that was there.
Turtle is the real thing, 144 tiles, the layout everybody recognises. Desk is 96 and fits a window. Pocket is 72 and eight tiles across, which is the widest board that still leaves a thumb-sized tile on a phone — it is picked for you on a small screen, and you can always switch.
The same board for everyone, one for each layout. Any finished daily counts, on any layout. Weekends never break a run. Missed a day? Play it the next day.