* Open the door for future browser history feature in #28536, which is currently not feasible when customize-loader is used.
* Remove customizer-loader from being used on admin screens for Dashboard, Themes, non-shiny theme install/update.
* Keep the customize-loader functionality available for plugins, for the time being. It may become deprecated.
* Ensure `return` param in customizer links in Themes screen update to reflect `search` updated by `pushState`.
* Persist `return` when reloading Customizer due to theme switch, autosave restoration, or changeset trashing.
* Use `location.replace()` instead of changing `location.href` when trashing.
* Hide theme browser while Themes screen is loading when there is a `search` to prevent flash of unfiltered themes.
* Use throttling instead of debouncing when searching themes to ensure that screen is updated immediately on page load.
* Fix encoding and decoding of `search` param between URL and search field.
* Add support for dismissing autosaves when closing customize-loader, when it is used by plugins.
* Skip sending changeset UUID to customize-loader for population in browser location if changeset branching is not enabled.
See #28536.
Fixes#40254.
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