Site migration: Show warning when user try to exit the instructions while the process is still happening

[gabrielcaires]

Closes #93557

Proposed Changes

• Show a warning message if user tries to close the page while the migration is still happening

Why are these changes being made?

• Nowadays, the site preparation for migration requires the user to wait until all processes are finished.

Note

We can't customize the message or show a custom UI due to browser security restrictions

Testing Instructions

• Go to any of the migration flow (E.g /setup/migration)
• Follow all steps for "I ll do for myself" until you reach the instructions page
• Try to reload the page while the migration spinner is still happening
• Check if you can see a message (We CAN"T customize the message due to browser security restrictions)
• Wait until the migration processing is completed
• Try to reload the page again and check if there is no message

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-122101

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-122101&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-122101&env=a8c-for-agencies

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/show-warm-message-unload on your sandbox.

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[fditrapani]

Looks great on the front-end. Thanks for the update!