[META] twig debug can break parts of site

@jwilson3 The issue you added was already 2nd in the list.

Whoops! Thanks for catching that and the cleanup

Chiming in here and adding another case where this causes problems: 🐛 newline on glossify-link.html.twig causes extra space Active .
I also had this in a custom theme when checking whether a field is empty (which it was, but not when the debug code inserts newlines).

This does not only cause problems in non-HTML output as the RSS output might imply.

All of my problems got fixed by the following patch and I think it does more good than bad, so I think it could be included in core.

Current firefox and chromium add newlines to comments in developer tools anyway, so this does not really cause any disadvantage for developers, I think, and it really fixes problems on a lot of places (in my case).

Applies against Drupal 10.1

--- /core/themes/engines/twig/twig.engine.org   2024-05-08 16:37:50.990994524 +0000
+++ /core/themes/engines/twig/twig.engine   2024-05-08 16:38:47.900186459 +0000
@@ -63,8 +63,8 @@
     throw $e;
   }
   if ($twig_service->isDebug()) {
-    $output['debug_prefix'] .= "\n\n<!-- THEME DEBUG -->";
-    $output['debug_prefix'] .= "\n<!-- THEME HOOK: '" . Html::escape($variables['theme_hook_original']) . "' -->";
+    $output['debug_prefix'] .= "<!-- THEME DEBUG -->";
+    $output['debug_prefix'] .= "<!-- THEME HOOK: '" . Html::escape($variables['theme_hook_original']) . "' -->";
     // If there are theme suggestions, reverse the array so more specific
     // suggestions are shown first.
     if (!empty($variables['theme_hook_suggestions'])) {
@@ -106,17 +106,17 @@
         $prefix = ($template == $current_template) ? 'x' : '*';
         $suggestion = $prefix . ' ' . $template;
       }
-      $output['debug_info'] .= "\n<!-- FILE NAME SUGGESTIONS:\n   " . Html::escape(implode("\n   ", $suggestions)) . "\n-->";
+      $output['debug_info'] .= "<!-- FILE NAME SUGGESTIONS:\n   " . Html::escape(implode("\n   ", $suggestions)) . "-->";

       if (!empty($invalid_suggestions)) {
-        $output['debug_info'] .= "\n<!-- INVALID FILE NAME SUGGESTIONS:";
-        $output['debug_info'] .= "\n   See https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!theme.api.php/function/hook_theme_suggestions_alter";
-        $output['debug_info'] .= "\n   " . Html::escape(implode("\n   ", $invalid_suggestions));
-        $output['debug_info'] .= "\n-->";
+        $output['debug_info'] .= "<!-- INVALID FILE NAME SUGGESTIONS:";
+        $output['debug_info'] .= "   See https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!theme.api.php/function/hook_theme_suggestions_alter";
+        $output['debug_info'] .= "   " . Html::escape(implode("\n   ", $invalid_suggestions));
+        $output['debug_info'] .= "-->";
       }
     }
-    $output['debug_info']   .= "\n<!-- BEGIN OUTPUT from '" . Html::escape($template_file) . "' -->\n";
-    $output['debug_suffix'] .= "\n<!-- END OUTPUT from '" . Html::escape($template_file) . "' -->\n\n";
+    $output['debug_info']   .= "<!-- BEGIN OUTPUT from '" . Html::escape($template_file) . "' -->";
+    $output['debug_suffix'] .= "<!-- END OUTPUT from '" . Html::escape($template_file) . "' -->";
   }
   // This output has already been rendered and is therefore considered safe.
   return Markup::create(implode('', $output));