Autosaving while offline (sessionStorage)

[swissspidy]

Feature Description

When the user is editing a story while being offline, can we store autosaves in sessionStorage like Gutenberg does?

Then we could display messages like "The backup of this post in your browser is different from the version below. Restore backup"

Alternatives Considered

Additional Context

Related: #3841 (Add message when Autosave exists)

Original Gutenberg implementation: WordPress/gutenberg#16490

Code is currently here: https://github.com/WordPress/gutenberg/blob/7d880708c6281c185256b8e7d3dffe178da33d09/packages/editor/src/components/local-autosave-monitor/README.md

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Acceptance Criteria

Implementation Brief

[swissspidy]

Current Gutenberg implementation:

https://github.com/WordPress/gutenberg/blob/d694d782feb0bed461efbf390f87b4baaf9b32f9/packages/editor/src/components/local-autosave-monitor/index.js

Technically it's rather simple:

1. The content is regularly saved to session storage.
2. Whenever a proper autosave to the db happens, session storage is purged
3. When loading the editor and there is something different in session storage, a notice is shown to the user

[swissspidy]

![Screenshot 2021-12-07 at 22 11 23](https://user-images.githubusercontent.com/841956/145122381-e2d3ded8-b3f1-499d-b538-8134c6844697.png


)

[felipebochehin87]

Tested against PR #12129.

1. Enabled auto save in Experiments
2. Started a new story
3. Didn't save changes
4. Visited the Dashboard and reopened the story - message was displayed
5. Clicked on Dismiss - unsaved changes were not loaded
6. Made changes, didn't save them, visited the dashboard and reopened the Story - message was displayed
7. Clicked on Restore Backup - changes were loaded
8. Made other changes, saved, visited the dashboard and reopened the story - message was not displayed