[3.10] Add a check whether we are running the core backend template and issue an info when that is not the case

[jgerman-bot]

New language relevant PR in upstream repo: joomla/joomla-cms#35107 Here are the upstream changes:

Click to expand the diff!

diff --git a/administrator/components/com_joomlaupdate/models/default.php b/administrator/components/com_joomlaupdate/models/default.php
index 1e8160fdb91f..bd6c803d5067 100644
--- a/administrator/components/com_joomlaupdate/models/default.php
+++ b/administrator/components/com_joomlaupdate/models/default.php
@@ -1799,4 +1799,63 @@ protected function translateExtensionName(&$item)
 		// Translate the extension name if possible
 		$item->name = strip_tags(JText::_($item->name));
 	}
+
+	/**
+	 * Checks whether a given template is active
+	 *
+	 * @param   string  $template  The template name to be checked
+	 *
+	 * @return  boolean
+	 *
+	 * @since   __DEPLOY_VERSION__
+	 */
+	public function isTemplateActive($template)
+	{
+		$db = $this->getDbo();
+		$query = $db->getQuery(true);
+
+		$query->select(
+			$db->qn(
+				array(
+					'id',
+					'home'
+				)
+			)
+		)->from(
+			$db->qn('#__template_styles')
+		)->where(
+			$db->qn('template') . ' = ' . $db->q($template)
+		);
+
+		$templates = $db->setQuery($query)->loadObjectList();
+
+		$home = array_filter(
+			$templates,
+			function($value)
+			{
+				return $value->home > 0;
+			}
+		);
+
+		$ids = JArrayHelper::getColumn($templates, 'id');
+
+		$menu = false;
+
+		if (count($ids))
+		{
+			$query = $db->getQuery(true);
+
+			$query->select(
+				'COUNT(*)'
+			)->from(
+				$db->qn('#__menu')
+			)->where(
+				$db->qn('template_style_id') . ' IN(' . implode(',', $ids) . ')'
+			);
+
+			$menu = $db->setQuery($query)->loadResult() > 0;
+		}
+
+		return $home || $menu;
+	}
 }
diff --git a/administrator/components/com_joomlaupdate/views/default/tmpl/default_preupdatecheck.php b/administrator/components/com_joomlaupdate/views/default/tmpl/default_preupdatecheck.php
index 346e0b968591..9b5ffcf18a0f 100644
--- a/administrator/components/com_joomlaupdate/views/default/tmpl/default_preupdatecheck.php
+++ b/administrator/components/com_joomlaupdate/views/default/tmpl/default_preupdatecheck.php
@@ -44,6 +44,16 @@
 	)
 );
 
+if (version_compare($this->updateInfo['latest'], '4', '>=') && $this->isBackendTemplateIsis === false)
+{
+	JFactory::getApplication()->enqueueMessage(
+		JText::_(
+			'COM_JOOMLAUPDATE_VIEW_DEFAULT_NON_CORE_BACKEND_TEMPLATE_USED_NOTICE'
+		),
+		'info'
+	);
+}
+
 ?>
 <h2>
 	<?php echo JText::sprintf('COM_JOOMLAUPDATE_VIEW_DEFAULT_PREUPDATE_CHECK', $this->updateInfo['latest']); ?>
diff --git a/administrator/components/com_joomlaupdate/views/default/view.html.php b/administrator/components/com_joomlaupdate/views/default/view.html.php
index 815bb5aafefc..c64170864a42 100644
--- a/administrator/components/com_joomlaupdate/views/default/view.html.php
+++ b/administrator/components/com_joomlaupdate/views/default/view.html.php
@@ -101,6 +101,7 @@ public function display($tpl = null)
 		$this->phpOptions             = $model->getPhpOptions();
 		$this->phpSettings            = $model->getPhpSettings();
 		$this->nonCoreExtensions      = $model->getNonCoreExtensions();
+		$this->isBackendTemplateIsis  = (bool) $model->isTemplateActive('isis');
 
 		// Disable the critical plugins check for non-major updates.
 		$this->nonCoreCriticalPlugins = array();
diff --git a/administrator/language/en-GB/en-GB.com_joomlaupdate.ini b/administrator/language/en-GB/en-GB.com_joomlaupdate.ini
index 655c45c43ee1..314d99cdc92d 100644
--- a/administrator/language/en-GB/en-GB.com_joomlaupdate.ini
+++ b/administrator/language/en-GB/en-GB.com_joomlaupdate.ini
@@ -79,6 +79,7 @@ COM_JOOMLAUPDATE_VIEW_DEFAULT_EXTENSION_WARNING_UNKNOWN="Unkown error"
 COM_JOOMLAUPDATE_VIEW_DEFAULT_EXTENSION_SERVER_ERROR="Update Server error"
 COM_JOOMLAUPDATE_VIEW_DEFAULT_EXTENSIONS="Extensions Pre-Update Check"
 COM_JOOMLAUPDATE_VIEW_DEFAULT_EXTENSIONS_NONE="No extensions installed."
+COM_JOOMLAUPDATE_VIEW_DEFAULT_NON_CORE_BACKEND_TEMPLATE_USED_NOTICE="We detected that you are not using a core admin template. You might find the upgrade process smoother if you <a href='index.php?option=com_templates&view=styles&client_id=1'>switch</a> to use the Isis template."
 COM_JOOMLAUPDATE_VIEW_DEFAULT_NON_CORE_PLUGIN_BEING_CHECKED="The system is currently checking these plugins to see if they could cause problems during the upgrade.<br><br>Please be patient while the checks are completed."
 COM_JOOMLAUPDATE_VIEW_DEFAULT_NON_CORE_PLUGIN_CONFIRMATION="Do you wish to ignore the warnings about potentially incompatible plugins and to proceed with the upgrade?"
 COM_JOOMLAUPDATE_VIEW_DEFAULT_POTENTIALLY_DANGEROUS_PLUGIN="Potential Upgrade Issue"