Positioning Portal elements

[sodiumjoe]

In implementation of ToolTip fade transitions, I ran into some issues. Here are some notes for future development that should make this kind of stuff easier:

• ContextMenu currently handles isExpanded (adds/removes dom nodes). This makes it difficult to implement transitions as a consumer of ContextMenu, because ContextMenu has to know when a transition is finished before it can remove the dom node, for example.

Possible solutions:

• build transitions into ContextMenu. This is not optimal as we'd like to be able to keep those concerns decoupled, to remain flexible on how a consumer of ContextMenu would like to apply transitions.
• build ContextMenu.FlyOut out of a react-motion element. The consumer could then pass any react-motion configuration props to ContextMenu.FlyOut. Unfortuntely, this would obviously hard-couple ContextMenu to react-motion.
• Pull positioning logic in ContextMenu out into utility functions. Components that need more flexibility than ContextMenu currently offers could simply compose on top of Portal and perform any positioning directly.

[ogupte]

I think the third solution is cleaner and more flexible.