Selecting a partially-visible tab scrolls but doesn't reveal arrow button

[Ortham]

This is reproducible on the demo page.

1. Click on the fourth tab of the first scrollable tabs example. It's the one labelled "THE ITEM FOUR" (but the "FOUR" is not visible).
2. The tab bar scrolls to the right until the whole of the fourth tab is visible, but the left arrow button remains hidden.

[bicknellr]

#129 fixes this.