## Overview Canvas renders structured content through `ContentTemplate` config entities, but a content creator cannot add free-form components to an individual entity. Per-entity tree editing was deliberately removed for 1.0 pending this feature (#3520487, #3541000). Exposed slots are the long-anticipated mechanism (ADR 0006, functional requirement 6): a site builder marks slots in a content template as editable per entity. This gives Canvas the hybrid structured + unstructured model that Paragraphs provides Drupal today, a structured entity with a free-form components area. The 1.x back end already ships scaffolding (an `exposed_slots` config property, `ValidExposedSlot` validation, a render-time subtree merge). The user-facing feature is missing. The community has proven demand and viability with a working fork (AlchemizeCanvas). ## Proposed resolution **An exposed slot is a field.** Each exposed slot is backed by its own `component_tree` field on the host bundle. The field machine name is the slot's stable identity, and per-entity slot content lives in that field as an ordinary component tree. This resolves the #3526189 spike ("one Canvas field per exposed slot") with the verdict: adopt. The template's `exposed_slots` map stores only the attachment point and label, declares config dependencies on the field configs, and detaches automatically when a field is deleted. Key changes: - **Manage slots from the template editor, non-destructively.** An "Expose slot" action either creates a new slot (label plus derived machine name, with copy stating it adds a field to the content type) or re-exposes an existing slot field (restoring content already stored in it, which is also how a slot is moved, per #3528458). Removing a slot is a detach that preserves the field and all entity content. Permanent deletion happens in Field UI behind Drupal's standard field-delete confirmation and batched purge (#3520517 becomes moot). - **Slots may be exposed empty or with content.** Template content already in the slot becomes the per-entity-overridable default; the slot-must-be-empty validation is relaxed accordingly. - **Multiple exposed slots per template.** The hard `assert(count === 1)` in `ContentTemplate::build()` is removed. - **Per-entity editing returns.** Entities of a templated bundle with exposed slots open in the Canvas editor. The Layout API serves the merged tree (template plus entity slot content) with per-component editability annotations. Template-owned components are locked; only exposed slots accept drops. A slot's default content shows until the editor overrides it (editing materializes an entity-owned copy). Overrides can be kept empty (stored as a shipped render-nothing marker so an empty field still means "inherit"), and reverting to the default is an explicit action. Writes are partitioned into per-slot subtrees and saved to each slot's field; writes touching template-owned components are rejected server-side. - **Rendering** merges each referenced slot field into the template tree, an override replacing that slot's default content. Content in detached fields is retained but not rendered (re-exposing restores it). - **Auto-save and preview** compose as elsewhere in Canvas: template drafts and entity drafts stay previewable without affecting the live site. - **BREAKING (storage contract, before any UI data exists).** Per-entity slot content moves out of a single shared Canvas field into per-slot fields. The shipped test-only shape (`parent_uuid` = template-internal component UUID) is abandoned. Entity data references nothing template-internal, so rebuilding a template never orphans entity content. Nothing released stores either shape (test fixtures plus one dev site only), so there is no update hook. ## User interface changes - **Template editor.** Slots are selectable; selecting a not-yet-exposed slot offers an "Expose slot" action, and an exposed slot shows usage statistics. The expose dialog has two paths: create a new slot or re-use an existing slot field. Removing a slot is a "Detach" confirm. - **Per-entity editor.** Template chrome is locked and non-interactive; exposed slots are the editable areas. A locked slot showing its template default offers "Unlock" (materialize an entity copy) or a jump to the template editor. The layers panel surfaces the content template as a shortcut to editing it. - **Contextual panel.** The entity form is split: "Page data" is trimmed to page-level metadata, and a new "Content" tab reaches the bundle's content fields (linking out to Drupal's edit form in phase 1, inline widgets in phase 2). - **Navigation.** A "Layout" entry point on templated entities launches the editor for that entity. Inside the editor a dedicated "Content" panel (its own left-rail icon) lists templated entities grouped by content type and searchable. The template editor links back to entities using it. --- _AI assistance was used to draft this issue summary, per Drupal.org's policy on the use of AI when contributing._