Steps:
1. Open more tabs that fix the notebook
2. Move to the last, so the first tabs are invisible
3. Start to close tabs
4. Tabs stay invisible even though there is space for them.
I can confirm this, even if I would not call a bug, it's a layout issue, but I will look into it, to see whether the behaviour can be changed.
I think it's more annoying, that after reopening a project, if the actual file is on a "far right" tab the tab does not gain focus (or more exactly it has the focus, but it is not visible).