ENGCOM-4522: #19835 Fix admin header buttons flicker #21791

Author: sidolov

app/code/Magento/Backend/view/adminhtml/templates/pageactions.phtml

@@ -8,7 +8,7 @@
 
 ?>
 <?php if ($block->getChildHtml()):?>
-    <div data-mage-init='{"floatingHeader": {}}' class="page-actions" <?= /* @escapeNotVerified */ $block->getUiId('content-header') ?>>
+    <div data-mage-init='{"floatingHeader": {}}' class="page-actions floating-header" <?= /* @escapeNotVerified */ $block->getUiId('content-header') ?>>
         <?= $block->getChildHtml() ?>
     </div>
 <?php endif; ?>

app/design/adminhtml/Magento/backend/Magento_Backend/web/css/source/module/main/_actions-bar.less

@@ -45,6 +45,10 @@
 .page-actions {
     @_page-action__indent: 1.3rem;
 
+    &.floating-header {
+        &:extend(.page-actions-buttons all);
+    }
+
     .page-main-actions & {
         &._fixed {
             left: @page-wrapper__indent-left;

lib/web/mage/backend/floating-header.js

@@ -48,6 +48,7 @@ define([
             this.element.wrapInner($('<div/>', {
                 'class': 'page-actions-inner', 'data-title': title
             }));
+            this.element.removeClass('floating-header');
         },
 
         /**