Don't play with the focus when handling focus event

[fcollonval]

References

Fixes #15407

Code changes

Create a new protected method setMode that allow to turn off some side-effect related to focus
Turn off focus actions when changing mode in focus event handlers.

User-facing changes

In full windowing mode, we can switch to any cell even if the active cell is not in the view.

Backwards-incompatible changes

None

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[fcollonval]

Need to test:

• Switching between lab tabs
• Switching between browser tab
• Dealing with focus within an output

[fcollonval]

js-notebook issue is legit:

  ● @jupyter/notebook › Notebook › #handleEvent() › focusout › should switch to command mode

    expect(received).toBe(expected) // Object.is equality

    Expected: false
    Received: true

      1358 |           MessageLoop.sendMessage(widget, Widget.Msg.ActivateRequest);
      1359 |           expect(widget.mode).toBe('command');
    > 1360 |           expect(widget.activeCell!.editor!.hasFocus()).toBe(false);
           |                                                         ^
      1361 |         });
      1362 |
      1363 |         it('should set command mode', () => {

      at Object.<anonymous> (test/widget.spec.ts:1360:57)

[krassowski]

Thanks! Conceptually and code-wise looks good. I will test locally tomorrow.
I think backport is not needed here and will change the milestone, but please correct me/revert milestone if I am wrong.

[krassowski]

Thank you!