Add function of panning the active tab's border

[ulivz]

When user switches a tab, it would be better to pan the border instead of changing the status directly

[egoist]

not sure what pan the border means

[ulivz]

@egoist Something like this:

[egoist]

I think this makes it unnecessarily complex.

[ulivz]

This implementation only need pure css, but having a defect is to increase the css's size.