From 6db8b736d0820bb7e082afa3b35633213bf90252 Mon Sep 17 00:00:00 2001
From: Dave Smith <getdavemail@gmail.com>
Date: Fri, 20 Sep 2024 11:13:47 +0100
Subject: [PATCH 1/3] Implement default algorithm

---
 .../src/store/private-selectors.js            | 20 +++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/packages/block-editor/src/store/private-selectors.js b/packages/block-editor/src/store/private-selectors.js
index 0e77e8e2ed433d..7c1a837d1667c9 100644
--- a/packages/block-editor/src/store/private-selectors.js
+++ b/packages/block-editor/src/store/private-selectors.js
@@ -16,6 +16,8 @@ import {
 	getTemplateLock,
 	getClientIdsWithDescendants,
 	isNavigationMode,
+	getBlocksByName,
+	getBlockAttributes,
 } from './selectors';
 import {
 	checkAllowListRecursive,
@@ -611,8 +613,22 @@ export function isZoomOutMode( state ) {
  *
  * @param {Object} state Editor state.
  *
- * @return {string|undefined} The section root client ID or undefined if not set.
+ * @return {string} The section root client ID.
  */
 export function getSectionRootClientId( state ) {
-	return state.settings?.[ sectionRootClientIdKey ];
+	const settingsRootClientId = state.settings?.[ sectionRootClientIdKey ];
+
+	// Specifically check that the setting was not provided to avoid
+	// cases where the provided setting is an empty string to signify
+	// the "root block" of the editor.
+	if ( settingsRootClientId !== undefined ) {
+		return settingsRootClientId;
+	}
+
+	return (
+		getBlocksByName( state, 'core/group' ).find(
+			( clientId ) =>
+				getBlockAttributes( state, clientId )?.tagName === 'main'
+		) ?? ''
+	);
 }

From 11fc7d5e7c44db0484ae496867a878f00394710f Mon Sep 17 00:00:00 2001
From: Dave Smith <getdavemail@gmail.com>
Date: Fri, 20 Sep 2024 11:14:31 +0100
Subject: [PATCH 2/3] Update setting to return undefined

---
 .../provider/use-block-editor-settings.js           | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/packages/editor/src/components/provider/use-block-editor-settings.js b/packages/editor/src/components/provider/use-block-editor-settings.js
index 12a67e44262b21..bbdd6a4a996b32 100644
--- a/packages/editor/src/components/provider/use-block-editor-settings.js
+++ b/packages/editor/src/components/provider/use-block-editor-settings.js
@@ -137,8 +137,7 @@ function useBlockEditorSettings( settings, postType, postId, renderingMode ) {
 			} = select( coreStore );
 			const { get } = select( preferencesStore );
 			const { getBlockTypes } = select( blocksStore );
-			const { getBlocksByName, getBlockAttributes } =
-				select( blockEditorStore );
+			const { getBlocksByName } = select( blockEditorStore );
 			const siteSettings = canUser( 'read', {
 				kind: 'root',
 				name: 'site',
@@ -148,15 +147,11 @@ function useBlockEditorSettings( settings, postType, postId, renderingMode ) {
 
 			function getSectionRootBlock() {
 				if ( renderingMode === 'template-locked' ) {
-					return getBlocksByName( 'core/post-content' )?.[ 0 ] ?? '';
+					return getBlocksByName( 'core/post-content' )?.[ 0 ];
 				}
 
-				return (
-					getBlocksByName( 'core/group' ).find(
-						( clientId ) =>
-							getBlockAttributes( clientId )?.tagName === 'main'
-					) ?? ''
-				);
+				// Allow default algorithm to determine the section root block.
+				return undefined;
 			}
 
 			return {

From e06a1c000ca0669b54bf927850f16ef347afdce8 Mon Sep 17 00:00:00 2001
From: Dave Smith <getdavemail@gmail.com>
Date: Fri, 20 Sep 2024 11:54:57 +0100
Subject: [PATCH 3/3] Return original default when it template-locked

---
 .../editor/src/components/provider/use-block-editor-settings.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/editor/src/components/provider/use-block-editor-settings.js b/packages/editor/src/components/provider/use-block-editor-settings.js
index bbdd6a4a996b32..da5558634e7a72 100644
--- a/packages/editor/src/components/provider/use-block-editor-settings.js
+++ b/packages/editor/src/components/provider/use-block-editor-settings.js
@@ -147,7 +147,7 @@ function useBlockEditorSettings( settings, postType, postId, renderingMode ) {
 
 			function getSectionRootBlock() {
 				if ( renderingMode === 'template-locked' ) {
-					return getBlocksByName( 'core/post-content' )?.[ 0 ];
+					return getBlocksByName( 'core/post-content' )?.[ 0 ] ?? ''
 				}
 
 				// Allow default algorithm to determine the section root block.