Managing the state and rendering of all the text items as the user manipulates them (this is on the desktop "type tester" homepage, not mobile) - essentially trying to build a "desktop-class" (to some degree!) experience à la Adobe Illustrator.

The rest of the site is pretty basic granted. Maybe it's not a great example, but it's the sort of thing that could have easily become unmaintainable in the old days as features were added quickly, but React and the component model (and MobX) made it much more manageable even as I hacked things in last minute ;)