Periodically (for no apparent reason), the water supply valve opens and "tops off" the quantity in the water closet. Can anyone tell me why this happens, and how (other than installing a new water level control) I can fix it? Many thanks, . . . Helper56
Replace the ball or flapper at the bottom of the tank. Over the years it could get hard and not seal well and you really have a slow leak down to the bowl which is the reason for topping off.
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.
Most likely, the fill-valve is doing what it is supposed to do, keep the tank water level at the proper height. I am guessing that you have a leak through the flapper. When the water level in the tank reaches a certain point, the fill valve replenishes the water. Try adding some food coloring to the tanks water and watch the bowl for an hour. If the water in the bowl changes color, you need to clean or replace the flapper. May also need a chain length adjustment. The chain only needs to allow the flapper to "just" seal.
If you have never made a mistake, you haven't done much.