Group header rendering may output markup as readable string

Problem/Motivation

Build a view with multiple levels of groups where not only the field value will be printed but also the field label. If the field content of one of those group fields contains markup, e.g. when theme debugging is turned on or a complex field like dates contain regular markup then that will be output as strong and not as markup - but only if you also output the field labels.

Steps to reproduce

Install Drupal 8, enable views and create a view with fields and multiple groups.

Proposed resolution

The issue is caused by \Drupal\views\Plugin\views\style\StylePluginBase::renderGrouping in line 595-598 you find this:

$group_content = $this->getField($index, $field);
if ($this->view->field[$field]->options['label']) {
  $group_content = $this->view->field[$field]->options['label'] . ': ' . $group_content;
}

The group content is markup originally and if the label is turned on, the string concat turns group content from markup into a string.

The solution should be to make this a renderable object with label being their own markup object or prepend the pure string label to the markup object.