Update the notebook tools on tracker.currentChanged

[jtpio]

References

Fixes #7643.

Code changes

Update the notebook tools on tracker.currentChanged.

User-facing changes

The notebooks tools should now be removed from the left area when a notebook that doesn't have the focus is closed:

Backwards-incompatible changes

None.

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[jtpio]

This will need a second look, to make sure there is no undesirable side-effect, and check if RestorablePool is the adequate place to implement the change.

[vidartf]

If possible, I would leave this to after strict null checks have been merged, as it should help highlight internal consumers not ready to handle the null value.

[jtpio]

OK to wait for #7657.

This can stay as a draft in the meantime.

[jtpio]

Actually the change in RestorablePool might not be strictly required, as the currentChanged signal seems to be emitted with the value of null when the last notebook is disposed and removed from the pool.

The issue for the notebook tools is that the handler for the tracker.currentChanged is not set up, only the one for labShell.currentChanged is:

jupyterlab/packages/notebook-extension/src/index.ts

Lines 506 to 514 in 636a1ae

	if (labShell) {
	labShell.currentChanged.connect((sender, args) => {
	updateTools();
	});
	} else {
	tracker.currentChanged.connect((sender, args) => {
	updateTools();
	});
	}

[jtpio]

I've updated the PR to only focus on resolving #7643.

There might still be a use case for being able to explicitly set pool.current to null, and have the currentChanged signal emitted with a null value. Although I'm not entirely convinced yet. In any case this could be discussed as a separate issue.

[blink1073]

@jtpio, this looks good as-is, or do you still consider it draft?

[jtpio]

It's ready, was just waiting for the tests to finish :)

[blink1073]

Thanks!