OMG poor kitty that must've been really awful to go through! Yup my cat is the same way follows me into the bathroom and is very affectionate for the whole time. if he doesn't follow me in right away and i close the door he will meow and scratch at the door constantly until i let him in. But i always put the lid down as soon as i get up to prevent him from falling in or playing with the water while it's flushing or drinking out of it. When he was a kitten he used to play with the toilet water and then go into the litter box still wet and omg what a mess! (weirdo cat that likes playing with water) So putting the lid down has become a rule of the house and if you forget to do so and the cat makes a mess you're cleaning it up. I hope this was a life lesson for you OP: never forget to put the toilet lid down right away ever again when the cat is around.....and i'd say this is more of a FYCL than yours.....poor kitty that must've been really awful to go through!

Generally cats don't like water and i don't think the cat wanted to actually jump in the toilet. Also a shower is very noisy and that combined with the water makes the cat more likely to panic. It's a big difference when a cat plays with water vs being immersed in water.