Tab bar does not scroll to show newly opened tab

[SimonSapin]

Steps to reproduce:

• Install and enable this extension
• Open enough tabs that the tab bar has a scroll bar
• Switch to the last (bottom) tab
• Middle click some link.

Nothing seems to happen. The tab bar needs to be scrolled manually to make the new tab visible. It would be nice if this would happen automatically when a new tab is opened.

In general, tabs closer to the bottom tend to be more recently opened and so more likely to be needed next. Scrolling to the bottom would ideally be "sticky": stay that way when more tabs are opened.