The fill valve needs a new $2.47 assembly cap
The part can be replace without tools in less than a minute.
The Korky Quietfill valve is quieter than a Fluidmaster and doesn't have water hammer like a Fluidmaster.
So, if I were you, I would just pop in the $2.50 part.
As far as the bowl goes, if you add water to a full bowl, any excess will drain out the trapway.
If you look at the top bend, that's as high as the water will ever get.
The water goes up over the dam, and down the drain.