React modal incorrectly restores input focus on unmount (React 16)

[denich]

Summary:

When conditional render is used - react modal restores focus on unmount without triggering onFocus event handler on target input.

Steps to reproduce:

1. Add <Modal />
2. Focus some input
3. Mount/unmount the modal
4. Focus restored w/o triggering onFocus event handler on the input

Expected behavior:

Input's onFocus event handler is triggered on focus restore.

Link to example of issue:

React16 and v3: https://codesandbox.io/s/6y6nx2np4k

Additional notes:

• It works in case of using isOpen attribute instead of conditional rendering
• It works with v2 and React15 in case closeTimeoutMS is set (I think v3 don't respect closeTimeoutMS on unmount anymore #530 issue prevents using this workaround)
• It works in case restore focus in timeout

In case of using for example draft-js as an input, this bug becomes a cause of the really strange behavior after focus restore.