Hi!
I use master branch (react-toolbox@1.3.4). The bug affects that version.

Bug description.

When the Tabs component contains some Tab components that vary depending on an app state, pointer element (under active tab) could be rendered with incorrect width.
This doesn't affect initial render, only render after state changes that affect Tab components.
E.g. if an active tab receives a new label prop that has longer or shorter text than current label then Tabs.updatePointer in Tabs.componentWillReceiveProps will calculate width at nextProps.index (which is correct), but based on old tab element (which is incorrect).
the first state example:
the second state example:
Notice how the blue pointer on the second example repeats the width of the pointer from the first example (previous tabs state).

Suggested solution.

1. In Tabs component remove componentWillReceiveProps method.
2. In Tabs component add componentDidUpdate method as shown in the pull request.

dev branch (2.x)

As I noticed, componentDidUpdate has been added like in my suggestion. That's nice but I think componentWillReceiveProps should be removed there. Though I cannot now test and prove for this version, it most probably triggers unnecessary render for the old tabs and pointer that will be rerendered by state change in componentDidUpdate anyway.