Comment Template: Improve UX of inner block selection (#38263)

* Comment Template: Rename firstBlock to firstComment

* Comment Template: Fix jumping of elements

* Comment template: Fix flicker

packages/block-library/src/comment-template/edit.js

@@ -1,15 +1,15 @@
 /**
  * WordPress dependencies
  */
-import { useState, useMemo } from '@wordpress/element';
+import { useState, useMemo, memo } from '@wordpress/element';
 import { useSelect } from '@wordpress/data';
 import { __ } from '@wordpress/i18n';
 import {
 	BlockContextProvider,
-	BlockPreview,
 	useBlockProps,
 	useInnerBlocksProps,
 	store as blockEditorStore,
+	__experimentalUseBlockPreview as useBlockPreview,
 } from '@wordpress/block-editor';
 import { Spinner } from '@wordpress/components';
 import { store as coreStore } from '@wordpress/core-data';
@@ -35,7 +35,7 @@ const TEMPLATE = [
  * @param {Array}  [props.comment]          - A comment object.
  * @param {Array}  [props.activeComment]    - The block that is currently active.
  * @param {Array}  [props.setActiveComment] - The setter for activeComment.
- * @param {Array}  [props.firstBlock]       - First comment in the array.
+ * @param {Array}  [props.firstComment]     - First comment in the array.
  * @param {Array}  [props.blocks]           - Array of blocks returned from
  *                                          getBlocks() in parent .
  * @return {WPElement}                 		Inner blocks of the Comment Template
@@ -44,7 +44,7 @@ function CommentTemplateInnerBlocks( {
 	comment,
 	activeComment,
 	setActiveComment,
-	firstBlock,
+	firstComment,
 	blocks,
 } ) {
 	const { children, ...innerBlocksProps } = useInnerBlocksProps(
@@ -53,15 +53,22 @@ function CommentTemplateInnerBlocks( {
 	);
 	return (
 		<li { ...innerBlocksProps }>
-			{ comment === ( activeComment || firstBlock ) ? (
-				children
-			) : (
-				<BlockPreview
-					blocks={ blocks }
-					__experimentalLive
-					__experimentalOnClick={ () => setActiveComment( comment ) }
-				/>
-			) }
+			{ comment === ( activeComment || firstComment ) ? children : null }
+
+			{ /* To avoid flicker when switching active block contexts, a preview
+			is ALWAYS rendered and the preview for the active block is hidden. 
+			This ensures that when switching the active block, the component is not 
+			mounted again but rather it only toggles the `isHidden` prop.
+			
+			The same strategy is used for preventing the flicker in the Post Template
+			block. */ }
+			<MemoizedCommentTemplatePreview
+				blocks={ blocks }
+				comment={ comment }
+				setActiveComment={ setActiveComment }
+				isHidden={ comment === ( activeComment || firstComment ) }
+			/>
+
 			{ comment?.children?.length > 0 ? (
 				<CommentsList
 					comments={ comment.children }
@@ -74,6 +81,44 @@ function CommentTemplateInnerBlocks( {
 	);
 }
 
+const CommentTemplatePreview = ( {
+	blocks,
+	comment,
+	setActiveComment,
+	isHidden,
+} ) => {
+	const blockPreviewProps = useBlockPreview( {
+		blocks,
+	} );
+
+	const handleOnClick = () => {
+		setActiveComment( comment );
+	};
+
+	// We have to hide the preview block if the `comment` props points to
+	// the curently active block!
+
+	// Or, to put it differently, every preview block is visible unless it is the
+	// currently active block - in this case we render its inner blocks.
+	const style = {
+		display: isHidden ? 'none' : undefined,
+	};
+
+	return (
+		<div
+			{ ...blockPreviewProps }
+			tabIndex={ 0 }
+			role="button"
+			style={ style }
+			// eslint-disable-next-line jsx-a11y/no-noninteractive-element-to-interactive-role
+			onClick={ handleOnClick }
+			onKeyPress={ handleOnClick }
+		/>
+	);
+};
+
+const MemoizedCommentTemplatePreview = memo( CommentTemplatePreview );
+
 /**
  * Component that renders a list of (nested) comments. It is called recursively.
  *
@@ -105,7 +150,7 @@ const CommentsList = ( {
 						activeComment={ activeComment }
 						setActiveComment={ setActiveComment }
 						blocks={ blocks }
-						firstBlock={ comments[ 0 ] }
+						firstComment={ comments[ 0 ] }
 					/>
 				</BlockContextProvider>
 			) ) }