Fix tags issue while alternating visual and text mode in the classic editor

syntaxhighlighter.js

@@ -1,5 +1,5 @@
-( function($) {
-	var shortcodes = window.syntaxHLcodes || 'sourcecode',
+( function( $ ) {
+	const shortcodes = window.syntaxHLcodes || 'sourcecode',
 		regex = new RegExp( '(?:<pre>\\s*)?(\\[(' + shortcodes + ')[^\\]]*\\][\\s\\S]*?\\[\\/\\2\\])(?:\\s*<\\/pre>)?', 'gi' );
 
 	window.syntaxHLescape = {};
@@ -8,24 +8,82 @@
 		return;
 	}
 
-	$( document ).on( 'afterPreWpautop.syntaxhighlighter', function( event, obj ) {
-		if ( obj.data && obj.data.indexOf( '[' ) !== -1 ) {
+	// Constants.
+	const $DOC = $( document );
+	const PRESERVE = 'PRESERVE';
+	const RESTORE = 'RESTORE';
+
+	// Tags that are removed by the core and their replacement to prevent being removed.
+	const tagsToPreserve = [
+		[ 'p', 'wp-p' ],
+		[ 'br', 'wp-br' ],
+	];
+
+	function replaceTag( code, from, to ) {
+		const tagRegex = new RegExp( `<(\/?)${ from }([>\\s\/]+)`, 'gi' );
+		return code.replace( tagRegex, `<$1${ to }$2` );
+	}
+
+	function replaceTagsToPreserve( code, action ) {
+		const indexReplaced = action === PRESERVE ? 0 : 1;
+		const indexReplacement = action === PRESERVE ? 1 : 0;
+		let newCode = code;
+
+		tagsToPreserve.forEach( function( tags ) {
+			newCode = replaceTag( newCode, tags[ indexReplaced ], tags[ indexReplacement ] );
+		} );
+
+		return newCode;
+	}
+
+	function preserveTags( code ) {
+		return replaceTagsToPreserve( code, PRESERVE );
+	}
+
+	function restoreTags( code ) {
+		return replaceTagsToPreserve( code, RESTORE );
+	}
+
+	function unescapeTags( code ) {
+		return code.replace( /&lt;/g, '<' ).replace( /&gt;/g, '>' ).replace( /&amp;/g, '&' );
+	}
+
+	const events = {
+		afterPreWpautop: function( event, obj ) {
+			if ( obj.data && obj.data.indexOf( '[' ) === -1 ) {
+				return;
+			}
+
 			obj.data = obj.data.replace( regex, function( match, shortcode ) {
-					return '\n' + shortcode.replace( /&lt;/g, '<' ).replace( /&gt;/g, '>' ).replace( /&amp;/g, '&' ) + '\n';
-				}
-			);
-		}
-	}).on( 'beforeWpautop.syntaxhighlighter', function( event, obj ) {
-		if ( obj.data && obj.data.indexOf( '[' ) !== -1 ) {
-			obj.data = obj.data.replace( regex, '<pre>$1</pre>' );
-		}
-	}).ready( function() {
-		$( '.wp-editor-wrap.html-active' ).each( function( i, element ) {
-			var id = $( element ).find( 'textarea.wp-editor-area' ).attr( 'id' );
-
-			if ( id ) {
-				window.syntaxHLescape[id] = true;
+				return '\n' + restoreTags( unescapeTags( shortcode ) ) + '\n';
+			} );
+		},
+		afterWpautop: function( event, obj ) {
+			if ( obj.data && obj.data.indexOf( '[' ) === -1 ) {
+				return;
 			}
-		});
-	});
-}( window.jQuery ));
+
+			const unfilteredCodes = obj.unfiltered.match( regex );
+			let i = 0;
+
+			obj.data = obj.data.replace( regex, function() {
+				// Replace by the unfiltered code piece.
+				const unfilteredCode = unfilteredCodes[ i++ ];
+				return `<pre>${ preserveTags( unfilteredCode ) }</pre>`;
+			} );
+		},
+		documentReady: function() {
+			$( '.wp-editor-wrap.html-active' ).each( function( i, element ) {
+				const id = $( element ).find( 'textarea.wp-editor-area' ).attr( 'id' );
+
+				if ( id ) {
+					window.syntaxHLescape[ id ] = true;
+				}
+			} );
+		},
+	};
+
+	$DOC.on( 'afterPreWpautop.syntaxhighlighter', events.afterPreWpautop );
+	$DOC.on( 'afterWpautop.syntaxhighlighter', events.afterWpautop );
+	$DOC.ready( events.documentReady );
+}( window.jQuery ) );