This greatly improves performance of previewing changes in the Customizer for non-
postMessage transport settings (JS-applied changes) by just refreshing the area of the page that has been changed. As such it eliminates some of the need to do
postMessage in the first place, while also reducing the amount of duplicated logic that would have to be implemented in JS to mirror what is being done in PHP. This resurrects some code from the old Widget Customizer feature plugin developed for 3.9. Writeup and feature plugin are available.
A low-level re-architecture of the Customizer plumbing that has a lot of side benefits and bugfixes, introducing some exciting possibilities for future feature plugins like scheduled settings, setting revisions, and drafted/pending settings. Partial Refresh is a dependency for this. Pull request available, but needs refresh. See proposal.
This is an important one for a client project I’m involved with, and so I’m having to prioritize it. I’m working on a client site that will have many users in the Customizer at a time, and given the way the Customizer is currently implemented (as with most areas of WP), there is no concurrency/locking support. So I’m working on adding locking at the control/setting level. See #31436.
(Cross-posted in comment.)