The software I found is placed in the status bar.

Be Focused pro (I think the biggest advantage of the paid version is that it has no ads and is cleaner, I bought the pro on Black Friday? 12)

You can choose the length of the countdown time and the subsequent status The bar is like this:

I will set the dock below to pop up and hide automatically. Double-clicking the window bar will make the window fill the screen. In the System Preferences dock settings

Turn off all notifications while reading and take a real mock test ~ The general situation is as shown below ~

Hope to adopt it