Issue #3294720 by abramm, nod_, alexpott: The attachBehaviors() for document...

Issue #3294720 by abramm, nod_, alexpott: The attachBehaviors() for document is only called after Big Pipe chunks are processed

core/modules/big_pipe/js/big_pipe.es6.js

@@ -93,6 +93,11 @@
       return false;
     }
 
+    // Attach Drupal behaviors early, if possible.
+    once('big-pipe-early-behaviors', 'body', context).forEach((el) => {
+      Drupal.attachBehaviors(el);
+    });
+
     once(
       'big-pipe',
       'script[data-big-pipe-replacement-for-placeholder-with-id]',

core/modules/big_pipe/js/big_pipe.js

@@ -47,6 +47,9 @@
       return false;
     }
 
+    once('big-pipe-early-behaviors', 'body', context).forEach(function (el) {
+      Drupal.attachBehaviors(el);
+    });
     once('big-pipe', 'script[data-big-pipe-replacement-for-placeholder-with-id]', context).forEach(bigPipeProcessPlaceholderReplacement);
 
     if (context.querySelector('script[data-big-pipe-event="stop"]')) {