I tested by changing " target="_blank ----> " target="_blank" , but it didn't work.
So, I change the source code of theme rendering.
Here are two examples
1.essential theme
file location : /moodle/theme/essential/classes/output/core_renderer.php
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$content .= html_writer::link($url, $menunode->get_text(),array('target' => '_blank'), array('title' => $menunode->get_title(),
'class' => $class));
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I just added ' array('target' => '_blank') '
2. adaptable theme
file location : moodle/theme/adaptable/renderers.php
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if (is_object($url) && (get_class($url) == 'moodle_url') && ($url->get_param('helptarget') != null)) {
$helptarget = $url->get_param('helptarget');
$url->remove_params('helptarget');
$content .= html_writer::link($url, $menunode->get_text(), array('title' => $menunode->get_title(),'target' => $helptarget));
} else {
$content .= html_writer::link($url, $menunode->get_text(),array('target' => '_blank'), array('title' => $menunode->get_title()));
}
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I just added ' array('target' => '_blank') '