This should now be fixed with the web version. I've only tested it on Firefox so let me know if you have any issues with a different browser. Enjoy!
The feature is very close to the original minesweeper, but as someone who plays almost exclusively with the both-click, I can tell you it’s way too finicky right now. In the original version you activate this mode when you press left and right click together regardless of when you release each one, the only way to cancel out of this mode is to drag the mouse to a blank space or entirely off the board before releasing both clicks.
I would much prefer that to a release left click first approach, as the number of times I speed mashed down a row and only half the numbers cleared properly really stopped me from enjoying this version. This is so close to a perfect recreation that I’m really hoping you’ll patch this one mechanic, because I really missed this game and I was so happy to find a faithful clone. It’s excellent right now, it’s just this one mechanic that is different to what I’m used to and expect.