Why does trist have a jump that breaks cc AND resets

It doesn't break CC. Very few types of crowd control cancel dashes.

Tristana and Ezreal's jumps both have cast times. As such, CCing them DURING the cast time but before they actually casted will cause the CC to take effect, but then for the jump to complete and happen. This is why you can get Blitzcrank Q'd or such and jump out.

Otherwise, currently all dashes in the game (as far as I know) are not stopped by hard CC (except airborne CC, such as knockups or knockbacks), instead applying the CC they took during a dash after the dash completes.

It's so fun! WHEEEEEEEEEEEEEEEEEEEEEEEE! :D

(Rito pls don't nerf)