This is the tab navigation. It has that animation. Nothing else does it (on Bubble) unless you add the CSS yourself.
For the transition, I’d suggest playing with the scroll percentage conditional with the height animation.
[Height: 0] | When scroll height > 90% : [ Height: 100 ]
Appearance > Transition Properties > Height (200ms Ease-in-out)
That’s a basic setup for something like that. There is better ways (with actual code of course), but this can work.