One "challenge mode" (entirely self-directed) for Minesweeper was to complete the board without flagging any mines. (Once you've revealed every non-mine space, all the mines get flagged automatically and you win.)

Disabling left+right click brings us a lot closer to that mode. Maybe you could formalize it and call it an option?

A much bigger problem with right click (and left+right click) is that it's bringing up the browser context menu. That shouldn't happen.

If we're trying to be authentic to original Minesweeper, I see that you've implemented the sunglasses face that appears once you've won the game. The original also uses an ":o" face (circular mouth) when revealing a hidden square. Maybe U+1F62E?