Modifying the "theme" object causes UI disruption

[peeveen]

This might just be a documentation issue.

I was experiencing some jittery UI funnies when using react-select controls near the bottom of the screen.
I eventually traced the problem to the theme property modifier function, even though all I was doing was changing one colour.

The problem is demoed here: https://github.com/peeveen/react-select-test

Basically, if I return a new object from the theme modifier function, then weird stuff starts happening.
But if I just alter some properties of the passed-in theme, and return the same object, it's OK.
Even I return a new object that is an exact copy of the theme (i.e. return {...theme}), it still happens.

So perhaps some docs indicating that the theme modifier function should return the same object?
Obviously if you would expect this to work, then I guess it's an actual bug.