Focus tracker does not focus on tab item

[brichet]

Description

Some context menu in Jupyterlab are not working correctly since #583.

It seems that the focus is not managed the same way when the tabs contain tabindex="0" attribute.

Reproduce

Update lumino to v2023.6.22 and right click on a Notebook tab in dock panel.

EXPECTED	ACTUAL

The currentwidget seems to be now the tabbar instead of the tab item, which breaks the behavior of the context menu.

Expected behavior

The focus correctly handling the tab item.

Reference

This bug has been found in jupyterlab/jupyterlab#14610

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[brichet]

Some more context after a little introspection:

Adding the tabindex attribute to tabs allows them to get focus (that's the point).
In Jupyterlab, the focusTracker tracks every element added to the shell, including the TabBar.
Since tabs never get focus, the TabBar could not be the current widget of the focusTracker.

Right-clicking on a selected tab should bring up the context menu for the widget displayed (the open document in the case of mainArea). This uses the focusTracker.current widget to choose whether or not to display certain options.
Now that the TabBar can receive focus, it becomes the current widget when a tab is right-clicked, which change the behavior of the context menu.