When this runs, $api.tabs has not been updated yet, so the calculation was not correct.
This caused various bugs like defaultIndex not working and aria-selected not being set, etc.
When porting, I thought I was being clever by just using ids instead of refs in $api.panels
But I didn't notice that the refs let you distinguish those panels which are rendered from those which are not.
My implementation had aria-controls= set for all tabs, even when their panels were not in the DOM.
Was previously using Vue names since Vue was the primary reference while porting, but as the public API is closer to the React one it makes sense to use that.
Fixes#8
