MC-35071: Unexpected behavior when closing tab in admin panel

Author: Viktor Sevch

app/code/Magento/Config/Block/System/Config/Form/Fieldset.php

@@ -175,6 +175,7 @@ protected function _getHeaderTitleHtml($element)
             '-link">' . $element->getLegend() . '</a>' .
             /* @noEscape */ $this->secureRenderer->renderEventListenerAsTag(
                 'onclick',
+                'event.preventDefault();' .
                 "Fieldset.toggleCollapse('" . $element->getHtmlId() . "', '" .
                  $this->_urlBuilder->getUrl('*/*/state') . "'); return false;",
                 'a#' . $element->getHtmlId() . '-head'