Move custom layout updates

design-documents/move-specific-layout-edit.md

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+### Move category, product and page specific layout updates
+Custom layout updates can be provided for specific categories, products and CMS pages via respective forms in the admin
+panel - this is how Merchants can customize entity-specific pages to include additional design and functionality.
+
+![alt text](img/custom_alyout_updates.PNG "Layout updates form")
+### The problem
+Layout updates are defined using XML and certain instructions often using PHP classes. Only developers can
+posses knowledge required to create a layout update yet the only way to provide these updates
+is via user forms more commonly used by content managers. That's the __1st problem__ - content managers don't
+need that functionality and it is better to hide it from them to avoid them from executing PHP code on the server.
+
+__2nd problem__ is that we do not allow developers to work with entity-specific layout updates in a developer way.
+Meaning that there is no way to use version control while editing these layout updates.
+
+### Solution
+Read custom layout updates from physical files on the server. Convert existing textarea _custom_layout_update_ field
+in category/product/cms page forms into a _select_ field where existing custom layout update files could be selected.
+
+To provide layout updates for admin users to select developers will create layout files following existing naming conventions for per-entity layouts
+\([page layout handles](https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/layouts/layout-overview.html)\)
+like _\<Magento_Theme_module_dir\>/view/frontend/layout/\(catalog\_product\_view\|catalog\_category\_view\|cms\_index\_index\)\_id\_\<entity ID\>\_selectable\_\<Layout update name\>.xml_.
+Special _selectable_ handle will be added for category/product/cms pages to pick up these files based on the selected by an admin
+update.
+
+Example: a file with the name _app/code/MyCompany/MyExtension/view/frontend/layout/cms_index_index_id_2_selectable_store1update.xml_ is created,
+then the select on the home page's edit form (home page has ID = 2) will look like this:
+
+![alt text](img/custom_layout_select.png "Layout updates form")
+
+This way merchants will still be able to manage different layouts for different stores or staging updates. In case
+of the example above the developer could also create _cms_index_index_id_2_selectable_anotherstoreupdate.xml_ and
+_cms_index_index_id_2_selectable_myjune2019update.xml_ to use for another store or for staging.
+
+This way only developers will be able to change layouts for specific entity pages and be able to use
+VCS while creating the layouts.
+
+### Availability to merchants without dedicated teams
+The described way to create these selectable updates requires creating a module/theme for a Magento installation which
+may make things a bit more difficult for merchants without dedicated teams comparing to being able to just copy-paste
+layout XML into the forms. However we could provide a detailed instruction with the link to it thoughtfully provided
+near the new selector to guide merchants on how to create these layout files step by step.
+
+### Backward compatibility
+In order to preserve backward compatibility existing layout updates will work but users will not be able to create new ones.
+When an entity already had a layout update provided via the text area the select will show _'\*Existing layout update\*'_.
+
+This proposals allows to remove the 'Custom layout XML' textarea from admin pages in the next patch release - 
+existing custom layouts will remain and content managers will still be able to change layouts for specific entities by
+creating files on their server and selecting a desired layout even without a development team.