Merge a template update in a theme with your customised version of that same template

[jameskoster]

Consider the following flow:

• Activate AwesomeTheme 1.0.
• Customise the Page template in the Site Editor.
• Update to AwesomeTheme 2.0, which includes changes to the Page template.

It would be nice if there was a way to pull the updates made to the bundled template into your customised version.

When the blocks remain the same (just ordering / attributes changed) it would hopefully be possible perform such a merge automatically. Here there are similarities with pattern transforms which we've explored before. It may be nice to have a before / after preview to confirm, perhaps with options that allow you to choose what to pull in; attributes, composition, meta, everything.

It's not entirely clear to me how far we can go with this approach, but my assumption is that there will be a point where automatic merging simply isn't possible. In these cases we might not surface the merge option in the UI at all, or alternatively offer some kind of 'conflict resolution' UI.

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As a side note, theme switches might be a consideration here. Should it be possible to merge a customised version of a template supplied by Theme A with one supplied by Theme B?

[SaxonF]

Could this behave in a similar way to reverting synced blocks/patters back to their library/global state?

[jameskoster]

I think this is slightly different as it is about pulling in updates rather than resetting. The 'clear customisations' action we have for templates / template parts can probably be considered for synced blocks though, once we have established a source of truth for those in the UI, and implemented overrides :D