One of the fun side games I've been playing is to try to beat the game with the fewest possible events, which means not flagging squares - it's definitely much harder!
That's definitely the crux of the problem! Although it would be cool to see the clock go backwards when you drag the slider, it's probably more accurate to keep the clock counting up so it reflects the overall time it takes to complete the game.
The perils of hiding the scrollbar by default (and also using a fixed:bottom element). I feel the modern push for clean design sometimes goes too far - removing the scrollbar definitely being one of those instances.
I passed my time in lockdown creating an Event Sourced version of the classic game Minesweeper! Be sure to try out the slider after a few moves, or if you click on mine.