Fix newlines disappearing inside PRE,TEXTAREA elements

Author: sirreal

src/wp-includes/html-api/class-wp-html-processor.php

@@ -1358,7 +1358,25 @@ public function serialize_token(): string {
 				break;
 
 			case '#text':
-				$html .= htmlspecialchars( $this->get_modifiable_text(), ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML5, 'UTF-8' );
+				$text = $this->get_modifiable_text();
+				if (
+					WP_HTML_Tag_Processor::TEXT_IS_WHITESPACE === $this->text_node_classification &&
+					'PRE' === $this->state->stack_of_open_elements->at( $this->state->stack_of_open_elements->count() - 1 )?->node_name
+				) {
+					/*
+					 * DO NOT DO THIS
+					 * This case is exceptional and we must peek at processor internals.
+					 */
+					$newline_was_erased = Closure::bind(
+						fn () => $this->skip_newline_at === $this->token_starts_at,
+						$this,
+						WP_HTML_Tag_Processor::class
+					)();
+					if ( $newline_was_erased ) {
+						$text = "\n{$text}";
+					}
+				}
+				$html .= htmlspecialchars( $text, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML5, 'UTF-8' );
 				break;
 
 			// Unlike the `<>` which is interpreted as plaintext, this is ignored entirely.
@@ -1427,6 +1445,19 @@ public function serialize_token(): string {
 				case 'STYLE':
 					break;
 
+				/*
+				 * Textarea elements ignore a leading newline. It is not present in `get_modifiable_text()`
+				 * because the HTML parser doesn't include it in the text node.
+				 * Serialization can always add a leading newline because it will be ignored by an
+				 * HTML parser.
+				 * This is especially important in case the text starts with a sequence of newlines.
+				 * If normalization removes a single leading newline, it would cause the
+				 * _significant_ following newline to be ignore by any later processing
+				 * or a browser.
+				 */
+				case 'TEXTAREA':
+					$text = "\n{$text}";
+					// Intentional fallthrough, TEXTAREA should have default handling.
 				default:
 					$text = htmlspecialchars( $text, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML5, 'UTF-8' );
 			}