Template part: prevent adding block in post editor or inside post tem…

…plate or content blocks (#44480)

* Reordering the filter so that any filters added int the block library get applied first, and become overridden by post editor initialization.
Renaming `can` to `canInsert` for clarity

* Updated comment

* Formatting inline JS comment betterer :P

packages/block-library/src/template-part/index.js

@@ -62,13 +62,13 @@ export const init = () => {
 		'blockEditor.__unstableCanInsertBlockType',
 		'removeTemplatePartsFromPostTemplates',
 		(
-			can,
+			canInsert,
 			blockType,
 			rootClientId,
 			{ getBlock, getBlockParentsByBlockName }
 		) => {
 			if ( blockType.name !== 'core/template-part' ) {
-				return can;
+				return canInsert;
 			}
 
 			for ( const disallowedParentType of DISALLOWED_PARENTS ) {

packages/edit-post/src/index.js

@@ -80,22 +80,6 @@ export function initializeEditor(
 	settings,
 	initialEdits
 ) {
-	// Prevent adding template part in the post editor.
-	// Only add the filter when the post editor is initialized, not imported.
-	addFilter(
-		'blockEditor.__unstableCanInsertBlockType',
-		'removeTemplatePartsFromInserter',
-		( can, blockType ) => {
-			if (
-				! select( editPostStore ).isEditingTemplate() &&
-				blockType.name === 'core/template-part'
-			) {
-				return false;
-			}
-			return can;
-		}
-	);
-
 	const target = document.getElementById( id );
 	const reboot = reinitializeEditor.bind(
 		null,
@@ -137,6 +121,26 @@ export function initializeEditor(
 		} );
 	}
 
+	/*
+	 * Prevent adding template part in the post editor.
+	 * Only add the filter when the post editor is initialized, not imported.
+	 * Also only add the filter(s) after registerCoreBlocks()
+	 * so that common filters in the block library are not overwritten.
+	 */
+	addFilter(
+		'blockEditor.__unstableCanInsertBlockType',
+		'removeTemplatePartsFromInserter',
+		( canInsert, blockType ) => {
+			if (
+				! select( editPostStore ).isEditingTemplate() &&
+				blockType.name === 'core/template-part'
+			) {
+				return false;
+			}
+			return canInsert;
+		}
+	);
+
 	// Show a console log warning if the browser is not in Standards rendering mode.
 	const documentMode =
 		document.compatMode === 'CSS1Compat' ? 'Standards' : 'Quirks';