Pull Request for Issue #12787

Summary of Changes

Issue is due to the change at root/libraries/cms/application/site.php line 355 from:
$temp = new Registry; $temp->loadString($menu->params);

to

$temp = new Registry($menu->params);

Testing Instructions

Test the menu item params override no more taking effect in com_content, etc

Documentation Changes Required

Steps to reproduce the issue

Use JApplicationSite getParams('com_mycomponent') method for a certain component in the frontend, having a valid routed menu item.

Expected result

All component params are merged with the specific menu item parameters in the same JRegistry object
If you have a menu item parameter 'myparam' and you call:
$registry = $this->app->getParams('com_mycomponent') ;

You always had:
$myParam = $registry->get('myparam');

Actual result

Menu item parameters are not merged as properties of the JRegistry object that can be obtained using the JRegistry 'get' method, but instead menu items parameters are all included in an array property named 'data'.
If you have a menu item parameter 'myparam' and you call:
$registry = $this->app->getParams('com_mycomponent') ;

You have:
$menuParameters = $registry->get('data');
$myParam = $menuParameters['myparam'];

System information (as much as possible)

Issue is due to the change at root/libraries/cms/application/site.php line 355 from:
$temp = new Registry; $temp->loadString($menu->params);

to

$temp = new Registry($menu->params);

Currently menu item params are not applied in J3.7 at all. Just test creating for example a menu item for a single article view.