* Autosaved changes made on top of `auto-draft` changesets get written on top of the `auto-draft` itself, similar to how autosaves for posts will overwrite post drafts.
* Autosaved changes made to saved changesets (e.g. `draft`, `future`) will be placed into an autosave revision for that changeset and that user.
* Opening the Customizer will now prompt the user to restore their most recent auto-draft changeset; if notification is dismissed or ignored then the auto-draft will be marked as dismissed and will not be prompted to user in a notification again.
* Customizer will no longer automatically supply the `changeset_uuid` param in the `customize.php` URL when branching changesets are not active.
* If user closes Customizer explicitly via clicking on X link, then autosave auto-draft/autosave will be dismissed so as to not be prompted again.
* If there is a changeset already saved as a `draft` or `future` (UI is forthcoming) then this changeset will now be autoloaded for the user to keep making additional changes. This is the linear model for changesets.
* To restore the previous behavior of the Customizer where each session started a new changeset, regardless of whether or not there was an existing changeset saved, there is now a `customize_changeset_branching` hook which can be filtered to return `true`.
* `wp.customize.requestChangesetUpdate()` now supports a second with options including `autosave`, `title`, and `date`.
* The window `blur` event for `customize.php` has been replaced with a `visibilitychange` event to reduce autosave requests when clicking into preview window.
* Adds `autosaved` and `branching` args to `WP_Customize_Manager`.
* The `changeset_uuid` param for `WP_Customize_Manager` is extended to recognize a `false` value which causes the Customizer to defer identifying the UUID until `after_setup_theme` in the new `WP_Customize_Manager::establish_loaded_changeset()` method.
* A new `customize_autosaved` query parameter can now be supplied which is passed into the `autosaved` arg in `WP_Customize_Manager`; this option is an opt-in to source data from the autosave revision, allowing a user to restore autosaved changes.
Props westonruter, dlh, sayedwp, JoshuaWold, melchoyce.
See #39896.
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