Listen for events at mount point, not top level?

[petehunt]

Any event that occurs on the page (regardless of if it's on a DOM subtree managed by React) goes through React's event subsystem. It would be great if we instead listened at the mount point (and any child documents in e.g. iframes) instead of at the top of the page.

[zpao]

What if there are multiple mount points? Should we attach our suite of listeners at each mount point? Or is there a tipping point where we should instead just go top level?