Read-only mode results in data loss

[ykazakov]

Description

There appear to be several issues with the "read-only mode" enabled after clicking on the "clock" symbol in the status area:

1. If the notebook did not have any changes, e.g., if it was freshly created, the read-only mode cannot be disabled because the slider cannot be moved.
2. When switching to another tab/document while the read-only mode is active and switching back, the timeline slider disappears. Which makes it hard to recognize that the document is in read-only mode.
3. Upon closing the notebook in read-only mode, a save dialog appears, however, saving changes have no effect. This has resulted in a data loss.

Reproduce

Context

$ pip list | grep jupyter
jupyter_client            8.6.3
jupyter-collaboration     4.1.0b0
jupyter-collaboration-ui  2.1.0b0
jupyter_core              5.7.2
jupyter-docprovider       2.1.0b0
jupyter-events            0.12.0
jupyter-lsp               2.2.5
jupyter_server            2.15.0
jupyter_server_fileid     0.9.3
jupyter_server_terminals  0.5.3
jupyter-server-ydoc       2.1.0b0
jupyter-ydoc              3.0.3
jupyterlab                4.5.0a1
jupyterlab_pygments       0.3.0
jupyterlab_server         2.27.3
jupyterlab_widgets        3.0.13

Proposal

1. Disable read-only mode upon the second click to the clock icon.
2. Disable the "Save" option in the closing dialog if the document is in read-only mode
3. Ideally, do not allow editing documents in read-only mode

[krassowski]

When switching to another tab/document while the read-only mode is active and switching back, the timeline slider disappears. Which makes it hard to recognize that the document is in read-only mode

We have a way to handle this for real read-only files in JupyterLab core with an "read-only" indicator shown in the toolbar and custom handler for save (which displays a tooltip) so probably the same logic be used here.

[krassowski]

If the notebook did not have any changes, e.g., if it was freshly created, the read-only mode cannot be disabled because the slider cannot be moved.

This is tracked in #419