What you also want to do is pour vinegar down the tube in the tank that water sprays into
That feeds into the "jets" the water gets into the bowl from. Overtime those ports will crystalize over and it's a huge hassle. flushing with vinegar that way will break it up before it fully clogs. And if some are clogged, the vinegar just soaks in there till the next flush
Just the flap and the gasket it seats into, wipe it down with a wet paper towel, see if it still does it
You can also try adjusting the linkage (chain length) sometimes a flap "floats down" slowly because there's still some water. And that might be causing issues.
Anytime you fuck with a toilet, you want to try the least invasive methods first and work your way up.
Shit can get out of hand quickly and you may end up replacing everything and still experiencing the same issue.
You might want to say which part of the planet you're in. Americans just assume everyone has the same medieval plumbing they do, but the vast majority of people don't
Usually because the flap in the tank is letting a small amount of water out into the bowl. After a while, it'll trigger the fill valve to top itself back up.
You can try lifting the flap and cleaning it and the drain hole to see if that gives it a better seal. (Turn the valve off so it's not trying to refill while you work, obviously).
If not that or if the flap is visibly degraded, then you can buy a replacement flap at pretty much any home improvement store. They're easy to replace: they just kind of clip on.
That's usually the cause and easy to check. Less often, the fill valve can be wonky and just kind of kick on by itself.
If that's the case, you'll hear another sound while it's filling where the excess water is going down the overflow drain inside the tank. That's a harder fix as you have to replace the whole assembly, but it's not too terrible; just more involved than replacing the flap.
You can replace the gasket/float, or you can try cleaning the probably moldy/algae seal topside and float underside off and seeing if that fixes the leak. I always drain the tank and clean the seal first when I get this issue, takes a minute and usually fixes or significantly reduces the frequency.
Oh I want to know the answer to this as well. Sometimes my toilet will flush itself while I'm having a shower, and if I'm not paying attention I get cooked.
The toilet flushes because the bowl is filling up. That's what causes it to flush. Why the bowl is filling up when you shower... I have no idea. Sounds bad though.
There is an explanation for this behavior that hasn't been mentioned. You might have a leak between the bowl and the floor. This is much more serious and can cause expensive damage so if you see evidence of water on the floor, that needs to be addressed immediately.