Some UI effects are very simple, yet difficult to achieve. I already solved the vertical slide dilemma with jQuery, but I needed to implement it for React Transition Group.
When items are entering a list, we want them to fade in and smoothly slide the surrounding elements away, instead of brutally shifting our interface.
Transitions work by easing from one initial value to an other defined one. The problem with vertical sliding is that most HTML element containers do not have a predefined height, but rather auto adapt this value to fit the content. Therefor, it is quite tricky to animate the entrance of randomly sized blocks without jumping stroboscopically into the margin, padding and box-sizing soup.
My approach is to use the CSSTransition component callbacks to animate a negative margin and slide our items from the top.