Add revisions of settings to the Customizer

[westonruter]

In #30937 the concept of a “Customizer Transaction” is introduced, wherein each Customizer session gets a transaction containing the settings that are modified and eventually saved (or not). The wp_transaction is a custom post type with the settings JSON stored in the post_content. Every time the Customizer is accessed, a new wp_transaction post is created with post_status=draft and then when saving it becomes post_status=publish. As such, this automatically becomes a revision history for all saved changes to the Customizer.

With the work on Transactions, however, there is no UI for accessing these revisions. There is no way to see previous revisions, to revert a revision, to recall a revision to continue changes.

I did do some work on a ​Settings Revisions plugin for a potential UI, where a new Customizer section is added to load a previous revision and to provide the “commit message” for the new one (see ​plugin screenshots). However, this plugin was written before transactions.

See also:
#21666 (Customizer reset/undo/revert)
#20692 (Handle unsaved changes in the customizer)
#28721 (Scheduled changes for the customizer)
#31088 (Post revisions in the customizer)

This would be a good feature plugin, especially extending transactions in #30937 with a UI.

Is this something that could be in scope for transactions or would it be better to build on after the underlying support is done? I'm envisioning adding a split dropdown arrow to the save & publish button that opens up options like scheduled changes and revisions, and other related things that transactions would enable.

UI would be out of scope for transactions, but the underlying backend infrastructure would be in place with the introduction of transactions. A setting revisions plugin would be a good feature plugin to propose for Core inclusion after transactions are in place.

Just noting that the revision UI proposed in the Additional CSS feature in #35395 can be adapted and used to be a global versioning system instead of just for CSS. :)

@folletto yes, Customize Changesets (#30937) actually result in a new revision of customizer settings for every publish. At the moment the changesets get automatically garbage-collected since there is no UI, but if a plugin adds revisions support to the customize_changeset post type, then published changeset posts will persist. Revision history for a given setting can then be extracted from those published customize_changeset posts.