Avoid using system clipboard in Notebook widget

[brichet]

This PR removed the use of system clipboard from the Notebook widget.

References

Related to #18136, and opened as WIP to help debugging.

Code changes

When a clipboard interaction (copy, cut or paste) in Notebook involves cells, the Notebook is aware of the interaction, and can (i) store the copied cells locally (on copy or cut interaction) or (ii) compare the previous copied cell to the pasted one (on paste interaction).
This allows to emit a signal when cells are pasted, whether the pasted cells come from the same Notebook or not, and if they were copied or cut.

Before this PR, to store the copied/cut cells or to compare the pasted cells, the Notebook was using the system clipboard as the last copied cells.
After this PR, when a clipboard interaction happens, the Notebook is aware of the cells involved in the interaction, and does not need the system clipboard.

User-facing changes

None

Backwards-incompatible changes

None

[jupyterlab-probot]

Thanks for making a pull request to jupyterlab!
To try out this branch on binder, follow this link:

[krassowski]

Looks good on Binder

[brichet]

Do we want to update this test, to test both settings ?
We could revert #18335 and add a tests set with the setting to true, only for copy, cut and paste actions. If it starts to fail again, then we'll remove it.

[krassowski]

Do we want to update this test, to test both settings ?

Yes, I think we should.

[brichet]

OK I added the tests.
BTW, sharding the regression tests is really nice 👍

[krassowski]

Thank you @brichet!

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 4.5.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 986a06ead3b426f0d15803af203c626c98681d5d

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #18474: Avoid using system clipboard in Notebook widget'

4. Push to a named branch:

git push YOURFORK 4.5.x:auto-backport-of-pr-18474-on-4.5.x

5. Create a PR against branch 4.5.x, I would have named this PR:

"Backport PR #18474 on branch 4.5.x (Avoid using system clipboard in Notebook widget)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.