INCLUDECSS and INCLUDEJS broken on all front controllers (app.php)

I just noticed on the current master branch, all uses of INCLUDECSS and INCLUDEJS are broken on any controller served via app.php.

They are broken because the path to the controller (i.e.: app.php/help) is being inserted into the path to the linked CSS and JS files, for example:

[Error] Failed to load resource: the server responded with a status of 404 (Not Found)

http://localhost/phpbb/app.php/help/styles/prosilver/template/forum_fn.js?assets_version=1660

[Error] Failed to load resource: the server responded with a status of 404 (Not Found)

http://localhost/phpbb/app.php/help/styles/prosilver/template/ajax.js?assets_version=1660

To reproduce, just go to the FAQ page @ Area51 (same goes for any extension with a controller) and look at your browser console error log.