1-) This application written with Flutter/Dart. Since I have to use (task needs GetX, it's an obligation) "GetX" which is another state management solution, you can see C++ in the Languages section.
2-) Sometimes after clicking long, when you watch the process, you may not see the some numbers, since it depends on the "time". If you click so fast, then normally you may not catch it with your eyes.
As you can see in the gif above, firstly you need to launch the application. Then, click the button, with the number of even numbers. For example, 2,4,6,100 etc. Why it works with even numbers? Because, aim of this application, recording the time between first and last click. So, you need to click to button to start recording, timing, and counting. Then, when you want, click again to stop. To see how many times clicked, just you need to press as long. After clicking as long to Float action button, you will see it will be red color. At this time, it uses the time which recorded, and it will start to count until the number of your click.